THE DEVELOPMENT OF THE BUD IN BOTRYLLUS

Abstract

The bud arises as a disc-like thickening of the anterior atrial wall, consisting of a small number of columnar cells transformed from the atrial epithelium, overlain by an equivalent area of unmodified epidermis. The polarity of the disc and subsequent organism is an extension of that of the parental tissue, with regard both to the antero-posterior and lateral axes. Development itself is fundamentally extremely simple and direct. After the completion of development there is a phase of functional activity and a phase of autolysis and dissolution. For any given time-temperature scale the duration of these last two phases is as specific and determined as that of the developmental phase. Of the two tissues constituting the bud disc the epidermis forms only more epidermis, though acquiring the form of a whole organism including the ventral stolonic outgrowth. The atrial component of the disc forms everything else. As the disc expands, by means of cell multiplication, it transforms progressively into a hemisphere and eventually into a hollow sphere attached by a narrow stalk to the parental tissue. Two folds develop anteriorly and divide the vesicle into two lateral and one median chamber. The lateral divisions represent the atrial chambers, the median the pharyngeal sac and from it three evaginations are formed representing the heart, neural mass, and intestine respectively. Later development is primarily an elaboration of these unit-regions. As an example, the formation, growth and differentiation of gill slits in the pharyngeal wall is described in detail. The essential pattern of the stigmata is apparent even before they become perforate. Each stage in their development is precisely correlated with specific stages in the development of the whole organism. The bud anlagen of the succeeding generation appear as discs in the anterior wall of the left and right atrial chambers at a specific stage in the development. This stage is that in which rows of stigmata, while not yet perforate, are represented by ridges or folds of the pharyngeal wall. At the time of perforation, the buds are approximately at the closed vesicle stage. The buds, in fact, are to be regarded as essential constituents of the organization pattern, appearing and developing in time and place in a manner strictly analogous to that of any other unit structure. The gonads segregate as a mass from the lateral walls of the bud at an extremely precocious period, even while the primary vesicle is in process of formation. Once segregated, they in turn develop as a seemingly independent unit structure. The testes show the final lobular form virtually as soon as sufficient cells are present for its expression. Ova, apart from the associated internal and external follicle cells, grow and differentiate without dividing. They mature finally at the same time as the spermatozoa which cease dividing and differentiate later than any other tissue of the bud. The development of each tissue is fundamentally the same. A period of cell multiplication is followed by a phase of final differentiation. This last phase may or may not include a period of cell enlargement, depending on the cell type to be formed. In the case of ova the multiplication phase is barely present at all and the second phase occupies most of the developmental period, involving enormous growth. In spermatozoa the case is reversed and the final phase is extremely brief and actually involves reduction in cell size. Muscle tissue lies between these two extremes, while most other tissues approach more the condition of spermatozoa. The whole development of the bud and that of its component parts is therefore as direct a process as can be conceived, without there being any indication of the divergence to form tadpole larvae associated with egg development. Cell multiplication continues to a greatly varying extent in different parts and tissues, while the linear growth of the whole or of a non-dividing ovum follows a regular sigmoid curve typical of an approach to and attainment of a "steady state." In fact, the development of the bud is essentially such a unitary process that "wholeness" can be said to be the most outstanding feature of the organism not only in its final functional phases but of every moment of its existence, and especially of the beginning. It is virtually as though organization is present from the first, though the extent of its visible expression is closely correlated with and limited by quantity of available material at every moment of development.