712 SUMMARY OF CURRENT RESEARCHES RELATING TO 



There is practically little difference between the modes of yolk- 

 absorption in the chick and in the fish. 



The author brings forward facts to prove that there is between 

 ova, even of allied genera, considerable differences, and that at no 

 stage is there a positive identity. 



The mechanical construction, as it may be termed, of ova affects 

 the course of their development. The Teleost ovum has a relatively 

 enormous yolk, which must be included by the blastoderm in order 

 to be absorbed, and this relatively large yolk has much to do with 

 the difference observed between its development and that of a Mar- 

 sipobranch or Amphibian. The eggs of the Salmonidas have an 

 abundance of oil-drops in the vitellus, especially just under the 

 germinal disk. These by their buoyancy keep the disk constantly 

 directed upward. The cusk, the crab-eater, Spanish mackerel, and 

 moon-fish have eggs which are buoyant from the possession of a 

 single large oil-sphere situated almost exactly opposite to the germinal 

 disk, and thus keeping it face downwards — just the reverse of what 

 occurs in the Salmonoids. Even after hatching, the young are at first 

 unable to right themselves on account of the presence of the oil-drop. 

 The cod ovum has no oil-drop, yet floats with the germinal disk 

 downwards. That of Morone Americana (white perch-) is adhesive 

 and fixed with a very large oil-sphere, which keeps the disk on the 

 lower side of the vitelline globe. The shad egg is non-adhesive, and 

 heavier than water, and the germinal disk has a constant tendency to 

 arrange itself at the side of the vitellus as viewed from above, though 

 there is no oil to influence it. In Fundulus and Syngnathus the oil- 

 drops appear uniformly distributed. The number of proto-vertebrse or 

 primary somites differs so much that while Tylosurus has so many as 

 seventy-five pairs, Alosa has only eighteen to twenty. The author 

 ventures this bold remark : " When our knowledge is more complete, 

 we shall perhaps be able to distinguish the species apart by the eggs 

 alone, just as botanists have used the characters presented by seeds to 

 distinguish plants." 



Development of Viviparous Minnows.* — J. A. Eyder describes 

 the development of viviparous minnows, and particularly Gamhrusia 

 patruelis B. and G. The youug fish develope within the body of the 

 female parent, and within the follicles in which the eggs themselves 

 are developed. The follicles, which are covered with a rich network 

 of fine capillary vessels, assume the office of a respiratory apparatus, 

 by which the gases are interchanged between the embryo and the 

 parent fish. This follicle also acts as an egg-membrane, being 

 actually perforated by a round opening (" follicular pore ") which is 

 analogous to the micropyle of the ordinary fish-egg. The arrange- 

 ment of the follicles of the ovary within the body of the female is 

 described, and the peculiar differences between the two sexes in the 

 arrangement of the viscera pointed out. The fibrous bands, which 

 act as supports or stays to the basal portion of the anal fin of the 

 male, which is modified as an intromittent organ, are also described. 



* Science, iii. (1884) p. 7(59. 



