520 H. H. Ne%vman 



Stage 4 (54 hours) : The H pures showed a very uniform con- 

 dition, Hke {H4). The lens was well developed and the optic 

 cup was invaginating. The optic vesicles were clearly defined. 

 The primary interbrain was slightly lobed. There were on all 

 healthy specimens examined about twelve somites. A few very 

 much retarded specimens occur (Plate III, H4). 



The H hybrids were markedly behind the H pures. The 

 most advanced condition resembled closely (Hj), but the majority 

 were like (h^.). About 25 per cent resemble (Aj). The most 

 advanced specimens had only four or five somites (Plate III, h^). 



The M hybrids showed very little difference from the M pures so 

 far as the overgrowth of the germ ring and closure of the blastopore 

 were concerned, but the optic vesicles of the former were con- 

 siderably better developed than those of the latter. The condition 

 of the average M hybrid was scarcely more advanced than {hj), 

 (Plate III, m4.). 



M pures show a condition less advanced than M hybrids, between 

 (Aj) and (mj). M pures were from eight to ten hours behind H 

 pures at this stage (Plate III, M4.). 



Stage 5 (72 hours) : The H pures had taken a very marked 

 lead upon the H hybrids, for which the earlier establishment of a 

 heart-beat, with the accompanying vitelline and body circulation, 

 was probably responsible. The various brain lobes were large 

 and well defined. The optic cup was fully invaginated and the 

 lens spherical. The embryonic eye showed a considerable cavity 

 between the lens and the inside of the optic cup. Accompanying 

 the establishment of a circulation a large amount of pigment had 

 been laid down in the form of dark brown chromatophores, scat- 

 tered over the body and the yolk mass. On the latter they originate 

 in connection with the capillaries. The trunk had become opaque 

 with pigment. The embryos had, as a rule, quickened, wriggling 

 of the tail region being very noticeable. The rates of heart- 

 beat in ten specimens were as follows: 90, 98, 96, 92, 98, 97, 95, 

 98, 99, 95. Average, 95.9 beats per minute (Plate III, H^). 



The H hybrids (Plate III, h§) were not nearly so advanced as 

 the H pures, showing a condition only slightly more advanced 

 than that shown by the H pures at fifty-four hours {H^). Of 



