SEGMENTATION AND GASTRULATION 



127 



germ before the formation of micromeres or delaminatiou, except that the central 

 mrtritive yolk is here still undivided. 



The further development of the germ has unfortunately not yet been thoroughly 

 investigated. Some observations tend to show that at this point, as in the Gerxjonid 

 germ, delaminatiou or formation of micromeres takes place. The merocytes at the 

 surface divide in such a way that the outer portion separates off completely as a 

 nucleated micromere layer detached on all sides, while the inner portion remains in 



Fig. 97.— Segmentation and formation of blastoderm in the egg of an Insect. 



the nutritive yolk as a nucleated merocyte layer. Thus the two germ layers are 

 formed ; the outer micromere layer represents the ectoderm ; the central mass of 

 nutritive yolk with the merocytes which belong to it probably represents the 

 endoderm. 



The merocytes by increasing in number and feeding at the expense of the 

 nutritive yolk, become able at last to overpower and incorporate.it, i.e. they can 

 divide it. 



The gastmla is here a solid sterroplanula. 



It has been observed in many cases that in the partial segmentation, i.e. in the 

 multiplication of the central merocytes, only 

 some of them move towards the surface, there 

 to form the blastoderm, while the others 

 remain in the yolk. 



5. The meroblastic teloleoithal germ (Fig. 

 98). We left this germ at the 8-blastomere 

 stage, with undivided mass of nuti'itive yolk. 

 We may here also call the masses of proto- 

 plasm merocytes. They stand in exactly the 

 same relation to the collection of protoplasm 

 at the animal pole of the blastomeres of the 

 holoblastic telolecithal egg as do the merocytes 

 of the mesolecithal meroblastic germ to the 

 protoplasmic portion of the blastomeres of the 

 Ge'ryonid germ. 



The 8 primary merocytes divide in the 

 following manner. The 4 central merocytes 

 divide into 4 secondary central merocytes in 

 contact with one another at the animal pole, 

 and 4 secondary peripheral merocytes. The 

 former are now entirely severed from each 

 other and from the nutritive yolk as micro- 

 meres. The latter remain connected with the 

 yolk as merocytes. The primary peripheral 

 merocytes also divide, but their descendants remain connected with the yolk as 

 merocytes. Some of them even sink into the nutritive yolk as branched and probably 

 amoeboid merocytes. 



Fig. 98. — Segmentation of a mero- 

 blastic telolecithal egg (of a shark), after 

 Eiiclcert. mi, Micromeres ; dp, deutoplasm; 

 mc, merocytes. 



