II 



SEGMENTATION AND GASTRULATION 



123 



difficult in that divisions occur simultaneously at different points of the germ, and 

 the number of blastomeres considerably increases. Starting with the 8-blastomere 

 stage, we will further follow the course of development of each germ type. 



1. In the holoblastic alecithal germ (Fig. 91), out of the 8 blastomeres, by 

 repeated meridional and equatorial division, 16, 32, etc., blastomeres of about 

 equal size are produced, which together form the uni-lauiinar wall of a sphere, 

 which becomes hollow by the formation of a cavity (segmentation cavity, blastoccel) 

 (total equal furrowing). At this stage the germ is called blastula, or cceloblastula, 

 because it is hollow. During repeated division of the blastomeres the blastula 

 becomes flattened at the vegetative pole (O), the flattened part sinks into the seg- 

 mentation cavity more and more, so that the invaginated portion, which consists of 

 vegetative blastomeres, approaches the non-invaginated part by means of complete or 

 partial reduction of the cavity. We now have before us a germ consisting of two 

 layers of blastomeres, which have become epithelial. The outer layer is the ecto- 

 derm, the inner the endoderm. At the edge of the aperture of invagination or 

 blastopore the two layers pass into each other. The endodermal blastomeres or cells 



FIG. 01. Segmentation and gastrulation 

 of a holoblastic alecithal germ. B, Blastula. 

 D, gastrula. fh, Segmentation cavity ; e, ecto- 

 derm ; en, endoderm ; U, blastopore. 



FIG. 02. Segmentation and gastrulation of 

 a telolecithal egg with little nutritive yolk (of 

 Eupomatus). mi, Micromeres ; ma, macromeres ; 

 en, endoderm ; me, mesoderm ; U, blastopore. 



form together a hollow sac, the arch-enteron, the cavity of which opens externally 

 through the blastopore. At this stage the germ is called gastrula, and in this case 

 ccelogastrula, because the blastopore leads into an open enteric cavity. This pro- 

 cess, by which a blastula becomes a gastrula, is called invagination or embole. 



2. In the holoblastic telolecithal germs, the phenomena of blastula and gastrula 

 formation are at least apparently different according to the presence of much or 

 little nutritive yolk. Let us first consider a germ with little nutritive yolk, as, 

 for example, that of Eupomatus (Fig. 92). It consists of 4 animal micromeres 

 and 4 vegetative macromeres. The difference in their sizes is not very great, 

 because of the small mass of nutritive yolk, and the third plane of division lies near 

 the true equator. First the 4 micromeres divide, then the 4 macromeres. We thus 

 now have 8 micromeres and 8 macromeres. The micromeres continue to divide, and 

 the macromeres follow at a slower rate. While, however, the micromeres always 

 divide into two cells or blastomeres of pretty equal size, the equatorial divisions 

 of the macromeres are such that each divides into a micromere, which is directed 

 towards the animal side of the germ and contains less nutritive yolk, and into a 

 macromere, turned towards the vegetative pole and containing more nutritive yolk. 



