Mosaic Development in Ascidian Eggs. 1 79 



4. Posterior Half. Embryos {Figs. S3~5^)- 



All that has been said of the mosaic-like development of the 

 anterior half of the egg is equally true of the posterior half. The 

 cleavage progresses in normal fashion up to the time of the closure 

 of the blastopore. Figs. 53 and 54 represent posterior half em- 

 bryos of the 32-cell and 76-cell stages, respectively. The former is 

 entirely normal and the latter is normal in all respects save that a 

 single pair of cells, B^% is larger than in the normal embrv^o. The 

 clear, the yellov^ and the gray substances of the egg are distributed 

 exactly as in the posterior half of a normal embryo. The clear 

 ectoderm cells lie on the ventral side and only two of them appear 

 in the dorsal view shown in Fig. 54 (the two clear cells at the pos- 

 terior pole). In Fig. 53 the gray endoplasm is contained in two 

 cells (B^-^) and in Fig. 54, in four (B^-^ B^-^); these cells give rise 

 to the strand of caudal endoderm. The yellow crescent consists 

 at the 32-cell stage of a single arc of yellow cells (Fig. 53) which 

 then, by division, become a double arc of fourteen cells (Fig. 54); 

 the inner arc consists of eight mesenchyme cells and the outer of 

 six muscle cells. In all these respects these posterior half embryos 

 are entirely like the posterior half of a normal embryo. 



But while the pregastrular stages of these posterior half embr}'os 

 are like the normal, the gastrulae and later stages show many 

 interesting modifications. Figs. 55, 56, 57 are three views of one 

 and the same posterior half embry^o, the normal embryos of the 

 same stage being young tadpoles like Fig. 11. In all of these 

 figures the embryo is viewed from the dorsal side; Fig. 55 shows 

 the ectoderm cells which cover the dorsal surface; Fig. 56, the 

 muscle cells which lie below the ectoderm on the dorsal side; 

 Fig. 57 is an optical section at a still deeper level showing the 

 caudal endoderm and mesenchyme. Fig. 58 is another posterior 

 half embryo of similar age seen from the ventral side, showing the 

 yellow mesoderm cells on each side of the caudal endoderm. 



The gastrulation occurs between the stages shown in Figs. 54 

 and 55. The caudal endoderm and the surrounding arc of mesen- 

 chyme, shown in Fig. 54, invaginates; the muscle cells come to lie 

 above (dorsal to) the mesenchyme cells and finally the latter are 

 overgrown by the ectoderm in the manner shown in Fig. 8. In 

 normal embryos the posterior part of the blastopore is closed 

 chiefly bv the growth of the anterior lip; in the latter stages of 



