126 



COMPARATIVE ANATOMY 



CHAP. 



all remain telolecithal in the same way as in the 8-blastomere stage. The 32 

 blastomeres of the spherical germ form a single layer round a considerable central 

 cavity. A blastula-like stage thus occurs, though the germ has really another signi- 

 ficance, as the central cavity does not represent the segmentation cavity of the 

 alecithal germ, but, as we shall see, the enteric cavity. 



When 32 blastomeres are formed, the formation of micromeres follows. From 

 each blastornere a micromere is constricted off on the outer side, so that the germ 

 now represents a double -layered hollow sphere, whose outer layer is formed of micro- 

 meres, and whose inner layer consists of macromeres. The micromeres increase 

 in number (1) by themselves dividing, (2) by the formation once more of micro- 

 meres which are constricted off on the outer side of the macromeres. The micromeres 

 form the ectoderm, the macromeres the endoderm, which surrounds a completely 

 closed cavity the enteric cavity. The germ thus represents a ccelogastrula without 

 blastopore. We call this a cceloplanula. The formation of the two germ layers in 



FIG. 96. Segmentation and gastrulation of the Geryonid egg. mi, Micromeres ; 

 ma, macromeres ; e, ectoderm ; en, endoderm. 



the manner described above is called delamination. It must appear clear from our 

 description that this cannot be sharply distinguished from epibole. Both processes 

 rest on the formation of micromeres. In Bonellia and the Polydada the first 

 formation of micromeres, or delamination, takes place at the 4-blastomere stage, in 

 the Ctcnophora at the 8-blastomere stage, and in Geryonia at the 32-blastomere 

 stage. 



4. The meroblastic mesolecithal germ was left at the stage where the formative 

 yolk or protoplasm was divided into 8 small masses, each with a nucleus, in the 

 centre of the undivided nutritive yolk. If we compare this stage with the 8- 

 blastomere stage of the Geryonid germ, we shall see that these 8 masses of 

 formative yolk correspond with the 8 portions of protoplasm of the latter germ, 

 only that in the former, in consequence of the originally different arrangement of the 

 nutritive yolk, the masses of protoplasm lie at the centre, while in the latter they 

 are peripheral. In the meroblastic germ the nutritive yolk is undivided, the 

 formative yolk being incapable, at the time of division, of effecting the division at 

 the same time of so large a mass ; in the holoblastic egg the formative yolk controls 

 the whole less massive nutritive yolk. 



The 8 central nucleated masses of protoplasm of the meroblastic mesolecithal 

 germ (Fig. 97) we shall call merocytes. They are often branched, and have amoeboid 

 movements. Their processes penetrate the surrounding mass of yolk, and are also 

 connected with the thin layer of protoplasm which is found at the surface of the 

 germ. They feed at the expense of the deutoplasm. 



The 8 merocytes divide into 16, 32, and so on, and move at the same time 

 centrifugally through the yolk to the surface of the germ, where they form a simple 

 continuous layer. It is this layer of merocytes which is called the blastoderm. 

 The germ has now become centrolecithal, and agrees in its structure with the Geryonid 



