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granular protoplasm, can be distinguished from a yolk-laden 

 vegetative pole. The cleavage corresponds with this condi- 

 tion. At first the egg is divided into four large spheres by 

 cleavage along two meridional planes, but then four small 

 blastomeres are constricted off from these at the animal pole. 

 By division of the latter and the formation of new micromeres 

 from the macromeres, the number of the small cleavage 

 spheres rapidly increases ; they spread out over the four large 

 spheres, and finally envelop them completely, producing an 

 epibolic gastrula (Fig. 143 A). The small cells that are now 

 being constricted off from the macromeres no longer reach 

 the surface, but remain lying under the layer of micromeres. 

 They form the entoderm. Inside of these the four macro- 

 meres are still retained for a while. In the region of the 



Fig. 14S.—A and B, embryos of Bonellia (after Spengbl, from Balfour's Com- 

 parative Embryology). ^, epibolic gastrula; £, formation of the mesoderm ; hi, 

 blastopore ; ep, ectoderm ; me, mesoderm. 



blastopore there appears a layer of cells (Fig. 143 B), which 

 surrounds the blastopore in the form of a circle. Spengel 

 believes this to have arisen by a migration inward of the 

 micromere layer. He interprets it as the fundament of the 

 mesoderm (Fig. 143 B, me). 



Cleavage and the formation of the germ-layers in Thalas- 

 sema (Kowalevsky, Conn) do not take place in the same 

 manner as in Bonellia. In Thalassema cleavage is equal, and 

 its result is a blastula from which an invagination gastrula 

 arises. To be sure, the latter is not altogether typically ex- 

 pressed in Thalassema mellita, for the invagination merges 



K. H.E. X 



