522 H. H. Newman 



was as yet apparent in any specimen, in marked contrast to the 

 condition seen in the H pures where the pigmentation was readily 

 visible to the naked eye. The head region of the average H 

 hybrid was noticeably less advanced than that of the average H 

 pure (Plate III, h6). In some few specimens the embryos were 

 still as small and undeveloped as were the average H hybrids at 

 72 hours (/z5). These retarded specimens numbered about 10 

 per cent of the whole. 



The M hybrids were still in advance of the M pures, but the 

 difference was not so marked. In both of the majalis strains 

 the number of somites had increased to such an extent that it 

 was difficult to count them. The average size of the M hybrids 

 was a little greater than that of the M pures, but this difference 

 was not well marked. The M hybrids had begun to lag in devel- 

 opment and the M pures had almost overtaken them (Plate III, 

 m6y and M6). 



Stage 7 (96 hours) : The H pures had advanced chiefly in the 

 acquisition of a more definite body outline. Pigmentation had 

 become heavier and had rendered the body rather opaque, 

 making it difficult to see the outlines of the brain and nervous 

 system. The optic cavities were more pronounced and the 

 eye had become slightly pigmented (Plate IV, Hy). 



The H hybrids (Plate IV, hy) had grown to be a little larger 

 on the average than the H pures, but were comparatively pale, 

 only a very faint pigmentation being visible. This failure to 

 pigment at an earlier stage may be due to the fact that the paternal 

 species does not show any pigment until several days later than this 

 period. The heart-beats often numbered as follows: 126, 120, 

 118, 68, 112, 124, 120, no, 128, 116. Average 114.2 beats per 

 minute. This average is about seven beats to the minute faster 

 than the average often H pures of the same age. The rates of 

 heart-beat of all but one retarded specimen, with a rate of 68, are 

 considerably faster than those of the latter. The specimens with 

 beats of 126, 120, 124, 120, 128, were especially large and were 

 noticeably more heavily pigmented than the others, although none 

 of them were nearly so dark as the average H pure. 



The M hybrids, in general body form, seemed to be about on an 



