124 



COMPARATIVE ANATOMY 



CHAP. 



The division or 

 micromeres, which 



The number of micromeres thus increases, as in the first place they themselves divide, 

 and in the second place new micromeres, produced by macromeres, become added 

 to them. This process of the continual production of micromeres is extremely 

 important for the comprehension of all following types of segmentation and gastrula- 

 tion. The furrowing is total, but already a little unequal. The blastula which 

 results is a cceloblastula, with a cap-like cover of micromeres and a lower part 

 formed of a few yolk-containing macromeres. In consequence of the considerable 

 size of the macromeres the segmentation cavity is somewhat narrowed. The 

 gastrula is formed by invagination, and looks as if the layer of macromeres which sinks 

 in were grown round on all sides by micromeres. The gastrula is a coelogastrula, 

 and the arch-enteric cavity appears narrowed in consequence of the large size of 

 the macromeres. 



Closely connected with the segmentation and germ-layer formation just described 

 is another process, an example of which is afforded by the developing germ of Bonellia 

 (Fig. 93). The process is essentially the same ; the apparent variations are explicable 

 by the fact that the mass of nutritive yolk is more considerable. The 4 large 

 macromeres, burdened with yolk, appear in consequence of their size as the fixed 

 resting-point round which the processes of development take place. At first, 

 as before, the 4 micromeres divide. The 4 macromeres are telolecithal 

 blastomeres. The chief mass of the formative yolk left after the first division 

 or budding no longer lies in the direction of the animal pole, but at some 

 distance from it, towards the outer edge of the 4 micromeres. 

 budding of these 4 macromeres leads to the formation of 4 

 take up a position externally, side by side with those already formed (A). The 4 

 macromeres still contain a remnant of formative yolk, which, as it always lies at the 



edge of the micromere cap, moves along 

 the surface of the germ from the animal 

 side of the macromeres towards the vegeta- 

 tive pole of the germ. Thus the process 

 goes on (, C). As the micromeres divide, 

 and as their number increases by the con- 

 stant formation of new micromeres from 

 macromeres, the macromeres are at last 

 surrounded, everywhere except at a small 

 space at the vegetative pole. This process 

 is called epibole ; it is the growth of 

 micromeres over a resting mass of very 

 large macromeres. It will be seen from 

 the above description that this process and 

 that of invagination are fundamentally 

 identical. The gastrula which is formed 

 is a solid sterrogastrula, whose enteric 

 cavity is almost filled up by the large size 

 of the yolk-laden macromeres. The micro- 

 meres present at this stage form the ectoderm ; the macromeres represent the rudi- 

 ments of the endoderm and of a part of the mesoderm. The blastula stage of this 

 method of development is unrecognisable. 



From the type of segmentation exemplified by Bonellia we pass on to one nearly 

 related which occurs in the Polydada (Fig. 94). The formation of micromeres, and 

 their growth over the macromeres, is just as in Bonellia. But here the 4 micromeres 

 which are first constricted yield, by division, the whole ectoderm. The micromeres, 

 which become constricted off from the macromeres in the second (and third) order, 



FIG. 93. Segmentation and gastrulation 

 of the egg of Bonellia, after Spengel. mi, 

 Micromeres ; ma, macromeres ; ec, ectoderm. 



