GASTRULATION IN VARIOUS CHORDATA 



411 



the notochordal material. A similar distribution is effected in other regions 

 of the mesoderm. Therefore, axiation and antero-posterior extension of the 

 entodermal, notochordal, and mesodermal cells are conspicuous results of the 

 activities which effect emboly. 



2) Epiboly. The above description is concerned mainly with emboly, that 

 is, the inward migration of the notochord-mesoderm-entoderm-yolk complex. 

 Allied with these active events is the downward or caudal migration of the 

 blastoporal lips. This migration is illustrated in figure 191B-E. In this figure 

 it may be observed that, as the marginal zone cells of mesoderm and notochord 



Fig. 192. Movements of the parts of the blastula during gastrulation in amphibia. (Cf. 

 fig. 191.) (A) Results of epiboly. (Cf. fig. 191A-F.) Epidermal and neural areas envelop 

 the other areas during gastrulation. (B) Movements of the areas of the blastula during 

 emboly, as seen from the vegetative pole. Heavy arrows, solid and broken, show the 

 converging movements during emboly; light arrows show the extension and divergence 

 of cells after involution at the blastoporal margin (cf. fig. 191A-F). (C) Similar to 

 (B), as seen from the left side. 



PRIMARY EMBRYO 



MESODERM 

 NOTOCHOR D 

 NEURAL 

 T U B E \ > 



SECONDARY EMBRYO 

 MESODERM 



«#«;, *^"*^"~~'\'^?»:i'J^5«i^^ ARCHENTERIC 

 ^'^•^-*^'f^""|^# >(^V-- SPACE 



:T-- ■!^eSI«&.\ NOTOCHORD 



" V'^-W'^*^''-^ S ' ' * * - -r "^"V^^^^ NEURAL 



[ l*SP^V>j.V ^ . . y ■'. * ^■-■^'- *V»^^-f M E S D I 



m:. 



SECONDARY 

 EMBRYO 



PRIMARY EMBRYO 

 ' A ARCHENTERIC SPACE 



Fig. 193. Induction of a secondary embryo. (From Spemann, '38.) (A) Host embryo 

 shown in this figure is Triton taeniatus. A median piece of the upper lip of the blastopore 

 of a young gastrula of T. cristatus of approximately the same age as the host was im- 

 planted into the ventro-lateral ectoderm of the host. The implanted tissue developed into 

 notochord, somites, etc.; the neural tube was induced from the host ectoderm. (B) Cross 

 section through embryo shown in (A). 



