Mosaic Development in Ascidian Eggs. 1 83 



These blastomeres give rise only to those tissues and parts of an 

 embryo which would come from them normally. Nothing even 

 remotely resembling a complete normal larva is ever produced from 

 the anterior or posterior quadrants of the egg. 



5. Quarter Embryos {Figs. S9~7^)- 



The development of individual blastomeres of the 4-cell stage 

 furnishes additional confirmation of the mosaic theory as 

 applied to ascidian eggs; in every instance individual blastomeres 

 give rise only to those parts or organs which they would produce in 

 normal embryos. Quarter embryos generally show more abnor- 

 malities and variations than half embryos, — probably owing to the 

 more severe injury which they have suffered, which often affects 

 the surviving quarter of the egg. 



The cleavage of these quarter eggs is normal in every detail, 

 save that the position of the cells is sometimes slightly altered; 

 the rhythm of cleavage and the size and quality of the cells is the 

 same as in the corresponding quarter of a normal egg. In Fig. 59, 

 which corresponds to the i6-cell stage of the normal egg, each of 

 the surviving quadrants has divided twice; in Fig. 60 the left 

 posterior quadrant of a 44-cell stage is shown and in both of these 

 figures the size, quality and position of the cells as w^ell as the rhythm 

 of division and the distribution of the different ooplasmic sub- 

 stances is entirely normal. Fig. 61, which is the right anterior 

 quadrant of the 76-cell stage, is normal in every respect, save for the 

 position of the endoderm cells which are here displaced toward 

 the first cleavage plane. The mesoderm cells in the right poste- 

 rior quadrant, shown in Fig. 62, are not normal in position; the 

 two caudal endoderm cells (lying next the first cleavage plane) 

 are, however, normal and the ectoderm cells are normal save that 

 they show a tendency to grow inward at the first and second 

 cleavage furrows and thus surround the embryo. In particular, 

 attention should be directed to the yellow crescent and caudal 

 endoderm cells in Fig. 60, and to the neural plate and chorda arcs 

 in Fig. 61, which are similar in every respect to the quarter of a 

 normal embryo at these stages. 



I have already commented upon the fact that the quarter embryo 

 shown in Fig. 63 is a "false gastrula" since the invaginated cells 

 are ectodermal, probably neural plate cells, while the larger endo- 



