[95] EMBRYOGRAPHY OF OSSEOUS FISHES. 549 



development of pigment in an embryo fish, we have the very remarkable 

 case of Gambusia, in which i)igment-cells are developed in the skin, 

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

 in the young of ApeJfes, while the yonng fish is still inclosed in the 

 ovarian Ibllicle 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 i)igmented 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 Gamhusia is 

 due to the unsuppressed influence of heredity. 



In conclusion, it may be of interest to note that the young 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 pigmentcells 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 jjrobably during an early period of devel- 

 opment only. 



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



It is well known that in the large-yelked 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 

 many invertebrates. In the egg of the oyster only a slight eccentricity 

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

 depart but slightly from the holoblastic or even type of segmentation. 

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

 unequal, and therefore apjiroximates the meroblastic type more nearly 

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

 idacement of the nucleus or germinative vesicle before impregnation, 

 lead us to enunciate the follovring general principle: The niiddus or 

 (jerminative vesicle is permanently displaced from the center of the ovum in 

 proportion to the amount of food yelk ichich is developed^ the amount of its 

 eccentricity^ or the distance through which it is displaced from the original 

 center of the ovum is governed enMrely by the amount of food-yelh tchich is 

 stored in an egg during its intraovarian growth. This appears to be a 

 fundamental law of ovular development in general, and one which is 

 far-reaching in its significance in relation to the later phenomena of 



