IV. i INITIAL STRUCTURE OF THE GERM 237 



The segmentation of a ^ or of f right or left blastomeres is 

 partial exceptior the alteration in the direction of the furrows of 

 the fourth phase already described by Chabry. The blastomeres 

 divide as though the other half of the egg had not been killed 

 (Figs. 144, 146, 147). Eventually a half gastrula is formed with 

 exactly half the normal number of neural, chorda, endoderm, mesen- 

 chyme and muscle-cells ; the gastrula cavity is open on the side of 

 the dead blastomere. Many are, however, abnormal in that the 

 endodermand muscle- and mesenchyme cells do not invaginate (exo- 

 gastrulae) or showinvaginations of the ectoderm (pseudogastrulae). 



The half gastrula may develop into a half larva, with the 

 notochord, muscles, and mesenchyme lying on one side of the 

 medullaiy folds. The only signs of the regeneration of the 

 missing half are the overgrowth of the ectoderm on the naked 

 side, and the passing over of a few muscle-cells ; the full number 

 of the latter is, however, never found on both sides (Fig. 145 a-c}. 



The | larva is especially interesting, particularly when a pos- 

 terior blastomere has been killed, for here the anterior end is 

 complete with anterior mesenchyme on both sides ; the posterior 

 mesenchyme and muscles are, however, missing on either the right 

 or left (Fig. 145 d). The chorda and neural plate are stated to be 

 smaller than thenormal in these larvae, and there is no sense- vesicle. 



An embryo developed from the two anterior blastomeres of the 

 4- celled stage alone possesses the full number of neural, chorda, 

 anterior endoderm and anterior mesenchyme cells ; its hind end is 

 covered by ectoderm but shows no trace of muscles or posterior 

 endoderm or mesenchyme (Fig. 146 c, d}. 



The two posterior blastomeres produce an embryo devoid of 

 neural plate and notochord, of anterior mesenchyme and endo- 

 derm. By overgrowth of the ectoderm a sort of solid gastrula 

 is formed with a central mass of endoderm flanked by muscle- 

 cells and posterior mesenchyme. No tail appears (Fig. 147). 



A, i, and Y 1 ^ blastomeres also segment and develop partially. 

 An anterior i has half the proper number of neural, notochordal, 

 anterior endoderm and mesenchyme cells, but the neural plate 

 does not become folded and the chorda cells protrude irregularly 

 behind. A posterior quarter may gastrulate in the way described 

 for the posterior half, but exogastrulao and pseudogastrulae occur. 



