Hubbard.] * "* [Dec. 15, 



The Yolk Nucleus in Cymatogaster aggregatus Oibbons. 



By Jesse W. Hubbard * 



(Read before the American Philosophical Society, December 15, 1S93.) 



The so called yolk nucleus is a compact, irregular mass of granules 

 found in the yolk of the eggs of certain animals. It lies outside the germ- 

 inal vesicle and usually takes a deep stain similar to the nucleolus within 

 the germinal vesicle. 



It has been seen in the eggs of Cnidarians (Hacker and others), Nem- 

 atodes (Monlicelli), Sagitta (Hertwig), Crustaceans (Weismann and 

 others), Myriopods (Ludwig), Insects (Balbiani), Arachnids (Carus, Lud- 

 wig and others), Lamellibranchs (Ihering), Gastropods (Balbiani), Teleo- 

 steans (Van Bambeke), Batrachians (Cramer, Carus and others). Reptiles 

 (Elmer), Aves (Cramer and others). Mammals (Schafer). It is probably 

 a normal structure offish eggs, but on account of unfavorable conditions 

 its complete history has not been traced in any species. 



The problem we wish to solve is : (1) How does this body originate, (2) 

 what becomes of it, (8) what function has it in the egg? There are many 

 difficulties to the solution of the problem by ordinary fish eggs. Their 

 great size, complexity of structure owing to accumulation of yolk, close 

 proximity to each other and liability to crumble in sectioning — all are 

 a detriment to its study. 



Cymatogaster has in these respects many advantages over all others. 

 (1) The eggs of this genus are the smallest of fish eggs, hence a few sec- 

 tions show the entire egg. The eggs of CyiTiatogaster reach a maximum 

 size of .3 mm. in diameter; while other fish eggs average about 1 mm. 

 in diameter. (3) There are comparatively few eggs in each ovary, so they 

 are not distorted by crowding. (3) In their natural state, if uninjured by 

 reagents, they are perfectly spherical and thus admit of easy measure- 

 ment. (4) There is but little yolk when ripe and hence they may be sec- 

 tioned without crumbling. (5) The nucleus seldom contains many nu- 

 cleoli. It generally has but one distinct nucleolar spot. In very young 

 eggs, and those about ripe, a few others may be seen, not more than four 

 or five. (6) The last and most important advantage these eggs possess is 

 the fact that the yolk nucleus is conspicuous from the time the eggs have 

 reached a diameter of 20 ji until maturation and even beyond tliat to the 

 closing of tlie blastopore ! 



The observations were made on ovaries collected hj Dr. Carl Eigen- 

 mann on the coast of Calif )rnia. They represent individuals from 17 mm. 

 to 140 mm. in length and were preserved chiefly in June, July and 

 August and in October, November, December, January and February. 

 Most of them were preserved in Flemming's strong mixture of osmic- 



* Contributions from the Zoological Laboratory of the Indiana University, under the 

 direction ot Carl Eigenmann, iii. 



