38 The Chromatin in the Development of Hybrids 



of conditions become apparent. The blastoderm may continue to spread 

 in an apparently normal fashion, encompassing the yolk, and the em- 

 bryonic shield enlarges correspondingly. The " blastopore " closes and 

 the rudiments, of the embryo are laid down. The three germ layers, 

 the chorda and neural cord are differentiated (Fig. 2, Plate I). The 

 eyes were seen in only two out of all the specimens that were obtained. 

 Sections of one such embryo showed that the optic cup is forming and 

 the lens, composed of a mass of cells, is constricted off from the ecto- 

 derm (Fig. 3). No definite arrangement of the cells in the lens can be 

 raade out. In the retina the cells were arranged into more or less dis- 

 tinct transverse rows. In both structures the cell boundaries are seldom 

 to be made out; the nuclei, on the contrary, are large, distinct and pro- 

 vided with one or more very large nucleoli. 



The proportion of embryos that thus normally close the " blastopore " 

 is small. During the growth from the germ-ring stage to the closure of 

 the " blastopore " the failures in the developmental processes especially 

 show themselves in the variety of abnormalities which occur. The 

 embryo may stop at the early embryonic-shield stage and, after two or 

 three days of apparent life, die. Others endeavor to lay down the 

 embryo so that the normal processes may go on for a time. The blasto- 

 derm may more or less completely enclose the yolk with the result that 

 the embryos are too short in varying degrees and the " blastopore " may 

 remain as a long slit or an open cleft of varying form (Figs. 4, 5, 6, 7, 

 Plate I). The number of embryos dying at these varjdng stages is not 

 the same. Comparatively few die during the early embryonic-shield 

 stage. The bulk of the embryos starting on gastrulation succeed in en- 

 compassing the yolk to two-thirds or more of the extent giving rise to the 

 variety of " blastopore " formations above described. 



In the reciprocal hybrid the development beyond the cleavage stage is 

 markedly less successful. A large per cent of eggs will form the germ 

 ring and the early stages of the embryonic shield, but of these only an 

 occasional one closes the " blastopore " in an approximately normal man- 

 ner. The earlier stages of gastrulation not uncommonly form normally. 

 Beyond this the abnormalities occur. These are of the same general 

 character as those described in the Fundulus hybrid. Figures 8 to 10 

 (Plate II) show some typical cases. 



In both hybrids the developmental processes come to a standstill at 

 various stages during gastrulation and doubtless also during cleavage 

 stages. That the latter is true is evident from the fact that there are 

 always a number of eggs in cleavage that never form any germ ring. 

 From the table given on page 35, it appeared that the rate of development 

 of the hybrid eggs became increasingly slower than the normals as de- 



