IV. Embryology. 



45- 



The cells composing this mass are destined to degenerate later on^ and 

 to serve as food for tlie rest (food yolk). The invagination too, 

 occurs in a somewhat peculiar way : first of all, a fold is formed on 

 one side of the blastula (Fig. 34, 2), and gradually surrounds the 

 ovum ; then it grows down over the lower portion of the egg, so that 

 this comes to project into the archenteron. 



Fig. 34. —Formation of the gastrula in Amphibia, diagrammatic, longitudinal section. 

 1 Blastula. 2 The invagination has begun at i (the corresponding place in 1 is indicated 

 by an arrow) ; the invagination is in the form of a furrow, but does not yet surround the 

 egg. 3 The invagination is proceeding. 4 Perfect gastrula ; the archenteron is almost 

 filled with a projecting part of the hypoblast, which is later dissolved and absorbed by the 

 embryo, eh epiblast (light), en hypoblast (shaded), g gastrula mouth, h segmentation 

 cavity, i invagination furrow, w archenteron. — Orig. 



Tliis mode of gastrula formation is readily derived from the typical one ; and 

 indeed, simply results from the excessive thickening of the hypoblast cells. 

 If to the blastula figured in Fig. 29 4, there vcere added a large mass of 

 hypoblast cells, which took no active part in invagination, this would occur 

 as in Pig. 34. 



5. This is the kind of development which occurs in many Pish, 

 in Reptiles and in Birds. The egg-cell contains a large amount of 

 yolk, practically all the protoplasm is accumulated at one pole of the 

 egg, and it alone segments, while the greater part of the cell remains- 

 unsegmented; this is known as partial segmentation.. 



