6 INTRODUCTION 



is called total because the entire mass of the egg is separated 

 by the division into blastomeres, and it is called equal on 

 account of the approxiraatelj equal size of the resulting 

 blastomeres. The blastula-stage which, with large central 

 segmentation cavity, is produced by this cleavage, is called 

 a coelohlastida or arcliihlastuJa, whereas the gastrula arising 

 from the latter by a process of enfolding is called an inva- 

 giitation gastrula or embolic gastnda. 



In the eggs of some Cnidaria, especially Hydroids, whose 

 eai'liest development is accomplished exactly in the manner 

 described above — that is, by total and equal segmentation 

 and subsequent development of a coeloblastula — there exists 

 a method of entoderm formation (gastrulation) which differs 

 somewhat from the method by invagination just described, 

 although it can be reduced to the same. This is the forma- 

 tion of the entoderm by polar ingression. In this case the 

 entoderm does not arise by an invagination of the cells of 

 the vegetative pole, but the latter detach themselves from 

 the blastoderm and migrate into the blastocoele, which in 

 this way gradually becomes filled with a closely packed mass 

 of entoderm cells. It is only secondarily that the archenteric 

 cavity arises in this mass as a fissure, and that a mouth is 

 formed by dehiscence of the wall. One sees that this kind 

 of entoderm formation can readily be derived from that by 

 invagination, inasmuch as the essential difference from that 

 method of formation coinsists in the fact that the entoderm 

 cells give up their epithelial continuity at the beginning of 

 the ingrowth. 



Closely allied to the above-described type of total and 

 equal cleavage are forms in which a more or less con- 

 siderable amount of food^yolk is deposited in the vegetative 

 half of the egg. On account of this accumulation the vege- 

 tative portion of the egg considerably exceeds the animal 

 portion in mass. It follows from this that in the course of 

 cleavage, which here also is total, the cleavage cavity appears 

 relatively small, and occupies a very eccentric position near 

 the animal pole. The wall of the blastosphere, which can 

 still be called a coeloblastula, in this case presents a con- 

 sidei'able difference in thickness at the animal and vegetative 



