No. 2.] FORMATION OF THE FISH EMBRYO. 445 



thicker than for the normal, and looks much like a continua- 

 tion of the ectoderm of the central flattened plate. This con- 

 dition certainly suggests that this ectoderm of the germ-band 

 was destined for the central nervous system, but owing to the 

 lack of concentration of the nerve-plate it remained accumu- 

 lated in the germ-ring. 



The most remarkable modification that I have found is shown 

 in Fig. 34. The yolk is exposed only at the circular blasto- 

 pore (j). The blastopore is surrounded by a thick homogene- 

 ous ring. Beyond this, over the extra-embryonic region, is a 

 thin layer of cells. The cap of cells of the early blastoderm 

 must, therefore, have grown over the yolk equally on all sides 

 without the differentiation of any embryonic portion. If this 

 is true, we see why the germ-ring should be so enormously 

 thick. Sections were made through this embryo (Fig. 43). 

 They show that a large tongue of cells turns in around the 

 whole inner perimeter of the thickening (i.e., in the blastopore). 

 The tongue is deep and composed of a large number of cells. 

 It is much larger than the tongue of the germ-ring of normal 

 embryos. The ectoderm of the ring is quite thick, and thick- 

 est at its inner edge. The ring seems to be the same through- 

 out, and shows no bilaterality.^ 



In Figs. 37 and 38 is shown an embryo in which the germ- 

 ring on one side has failed to grow, while on the other side 

 (right) it has retained its normal relation to the embryo. 

 Nevertheless, the embryo seems to have elongated posteriorly 

 and preserved its bilateral structure.^ The darker portion of 

 the figures (to the left) shows the remains of the germ-ring of 

 that side. Sections through this embryo show that the meso- 

 derm in the anterior portion of the body is equally developed 

 on the two sides. In the middle and posterior portions there 

 is a smaller amount of mesoderm on the left (defective) side 

 than on the right (Fig. 42).^ This mesoderm of the left side 



1 LerebouUet has figured a similar abnormality, in which the ring has a thick 

 process from one point. Rauber has described an embryo similar to the one 

 figured. 



2 Unfortunately, the record of the previous history of this embryo was lost. 

 Whether it was the result of some artificial solution or not I do not know. 



3 The figure has right and left reversed. 



