The Process of Heredity C23 



equality with the M pures. Heart rates often gave the following: 

 84, 82, 94, 83, 90, 82, 86, 66, 90, 78. Average 83.5. The average 

 state of advancement was about equal to that of H pures at 72 

 hours {Hs), (Plate IV, rW/). 



The M pures (Plate IV, M7) appeared very like the M hybrids. 

 The heart rate, however, was very slow and feeble as yet, averaging 

 42 in len specimens. Evidently the heart rhythm in M hybrids 

 was established considerably earlier than in M pures. All embryos 

 of both majalis strains were at this time entirely devoid of pig- 

 ment. To the naked eye they appear very pale as compared with 

 the heteroclitus embryos. 



Stage 8 (114 hours): The past eighteen hours showed a marked 

 advance in the H pure embryos. They had rounded out and 

 now exhibited a typically fishlike form, with caudal fin well 

 defined, the whole body opaque, and the eyes pigmented. The 

 figure {H8) will show the general appearance better than a verbal 

 description. Large chromatophores were present on the head 

 directly over the brain and upon the surface of the eye. The yolk 

 mass was shrunken to a noticeable extent (Plate IV, H8). 



The H hybrids (Plate IV, h8) were on the average not so ad- 

 vanced as the H pures, exhibiting a condition more like that of the 

 H pures at 96 hours (//7),except that many of the //hybrids were 

 larger than the latter. The pigmentation was as yet considerably 

 lighter than in the pure strain. The heart rate of ten H hybrids 

 averaged 128 as compared with 112 for the H pures of the same 

 age. More retarded and ill-developed specimens were noticeable 

 than at any previous stage of development, probably on account 

 of the failure of many to establish a normal circulation. A num- 

 ber of such ill-developed embryos were no farther advanced than 

 the average H pure at 72 hours {Hj). On the other hand 

 there were a few rather precocious specimens that were nearly 

 as advanced as the average H pure of the corresponding stage 

 {H8). 



The majority of the M hybrids (Plate IV, m8) were in size and 

 general structure about like the M pures. In certain details, 

 however, they were very diff^erent. The majority were lightly 

 pigmented both on the body and the yolk. The heart rates often 



