224 



Embryogenesis: Preparatory Phases 



This occurs in the development of Nereis 

 and Crepidula, etc. 



The mechanism of gastrulation has ap- 

 pealed to many investigators who employ 

 a physicochemical approach to the problems 

 of development. Thus far, such attempts to 

 explain gastrulation have been limited al- 

 most exclusively to considerations of the 

 mechanism of infolding of the entoderm 

 to form the primitive digestive tract. Almost 

 nothing has been done in elucidating the 



mechanism of exogastrulation. Thus, this 

 apparently fruitfvil approach has not yielded 

 the anticipated results. 



Some of the mechanical factors accom- 

 panying invagination have been studied. 

 (1) It has been assumed generally that con- 

 tinued increase in number of cells by divi- 

 sion and growth in the wall of the blastula 

 results in a lateral pressure among the cells, 

 which causes an in-folding of the weakest 

 part of the blastula sphere. However, by 



Fig. 73. Left: Outline of normally invaginating gastrula. a, The originally active zone derived from the 

 central spot of the gastral plate, and p from the plastic zone of the plate, which formed the periphery of the 

 plate surrounding a; and r, the new active ring. 



Right: Diagram showing form of cells in neurulation. In the deformation which makes the furrow, the 

 individual cells (one shown isolated) have the form of truncated wedges, two sides of which are inclined to 

 each other. (Figures from A. R. Moore, '41.) 



physicochemical causes of epiboly or de- 

 lamination. However, since the invagination 

 of the gastrula is a process upon which all 

 considerations of tissue foldings could be 

 based, to learn how vegetal cells undergo 

 this invagination might supply the key to 

 similar processes (such as neurulation) in 

 the formation of other organs (see Fig. 73). 

 One possible approach to a study of the 

 mechanism of gastrulation might be assumed 

 to lie in the fact that the inpocketing of 

 the gastrula of the sea urchin (also of Am- 

 phibia) can be reversed into an out-pocket- 

 ing or evagination. Such evaginated embryos, 

 called exogastrulae, were produced by Herbst 

 (1892) by treatment of the eggs during early 

 segmentation with a small amount of lith- 

 ium salts added to sea water. The lithium 

 ion itself is nonspecific, since exogastrulation 

 can be produced also by a variety of other 

 agents, which so far as can be seen have 

 nothing in common with the lithium ion. 

 However, there has been, as yet, no satis- 

 factory physicochemical explanation of the 



this hypothesis, it is difficult to explain why 

 the thicker vegetal cells should first invagi- 

 nate. It has been noted also that invagination 

 could take place without increase in cell 

 number. (2) Rhumbler ('02) was probably 

 the first to suggest that in-folding resulted 

 from a change in shape of the vegetal cells. 

 Biitschli ('15) and Spek ('20) postulated a 

 differential swelling between the inside and 

 outside of the entodermal layer. This was 

 supposed by Spek to be due to a special 

 arrangement of lyophilic and lyophobic 

 colloids in the entodermal plate. (3) Asshe- 

 ton ('16) suggested that invagination might 

 be brought about if there were an attraction 

 between cells of the blastula with the great- 

 est force being exerted along a line passing 

 through the peripherally located nuclei. 

 Waddington ('40) has modified this idea 

 slightly to suggest a fibrization theory. Ac- 

 cording to this, fibrils passing through cell 

 boundaries produce the effect. No definite 

 evidence in favor of this view is as yet 

 available. (4) A fourth suggestion (Morgan, 



