[95] EMBRYOCRAPHY OF OSSPZOUS FISHES. 549 



development ofju'^inent) in an erabryo fit*h, we have tlie very remarkable 

 case of Gamhusia, in wliicli pigment-cells are (leveloi)e(l in tbe skin, 

 especially on the head, to a remarkable degree, or almost as densely as 

 in the young of Apeltes^ while the young fish is still inclosed in the 

 ovarian follicle of the mother. The conditions by which it is surrounded 

 in the follicle being especially unfavorable to the accession of light, 

 inclosed as it is within the more or less extensively jjigmented walls of 

 the abdomen of the parent, and we are driven to the extremity of sup- 

 posing that this prenatal pigmentation of the embryos of Gnmhnsia is 

 due to the nnsuppressed influence of heredity. 



In conclusion, it may be of interest to note that the youDg of Pare- 

 pMppus, which in other respects develop almost exactly like the Spanish 

 mackerel during the early stages, soon show a tendency to form a red- 

 dish pigment over the abdomen and remains of the yelk-sack, on the 

 third day after hatching. The reddish pigment-cells of this form are 

 often confluent and have long and complex interjoined processes, much 

 flattened, like pigment-cells generally. In the young of the same species 

 an inch in length, the future vertical bands of the adult are already 

 outlined in black. The red pigment seems therefore to have a larval 

 significance, and to be useful probably during an early period of devel- 

 0])ment only. 



25. — The law of displacement of the germinative vesicle. 



It is well known that in the largeyelked or meroblastic eggs of many 

 vertebrates and invertebrates there is a migration of the nucleus of the 

 egg at a late stage of ovarian development towards the surface before 

 the nascent ovum has left its follicle. It is noteworthy that, on the 

 other hand, it is only the small holoblastic or evenly segmenting ova 

 without a yelk which retain their nucleus nearly in the center to the 

 time and condition of emission from the ovary. Examples of this type 

 are presented to us in the mature ova of mammals, of Amphioxus, and 

 manj' invertebrates. In the c^g of the oyster only a slight eccentricity 

 of the nucleus is notable in the mature egg, and we find that its eggs 

 dei)art but slightly from the holoblastic or even type of segmentation. 

 In Xassa, a gasteropod studied by Bobretsky, the segmentation is more 

 unequal, and therefore ai)proximates the meroblastic type more nearly 

 than the egg of Ostrca. The observed facts with regard to the dis- 

 placement of the nucleus or germinative vesicle before imju-egnation, 

 lead us to enunciate the following general principle: The tmclifus or 

 germinative vehicle is permanently displaced from the center of the ovum in 

 proportion to the amount of food yclh ichich is developed, the amount of its 

 eccentricity, or the distance thron<jh which it is displaced from the original 

 center of the ovum is governed cnUrely by the amount of food-yelli which is 

 stored in an egg during its intraorarian groicth. This appears to be a 

 fundamental law of ovular development in general, and one v>'hich is 

 far-reaching in ils significance in relation to flic later ]»lien()niena of 



