72 SURVEY OF THE REPRODUCTIVE PROCESS 



Entochoncha mirabilis, a molluscous animal, this, ac- 

 cording to J. MuUer, is identical with the germinal vesicle, 

 which in that species never disappears. In other cases, as 

 Ascaris, it has been supposed to arise from the nucleus of 

 the vesicle. Some such connection has been assumed, in- 

 deed, even when the vesicle seems to disappear entirely, as 

 it does in most ova. It has been thought that the deli- 

 quescence may not extend to its whole contents, and that 

 the " embryo-cell" may originate from some residuary por- 

 tions, as from some of the clear spherules, before referred 

 to, in the ova of Birds and Batrachia. In the Hydrozoa, 

 according to Gegenbauer, at least in the Corynidae, Calyco- 

 phoridse, and Physophoridse, " the germinal vesicle" does 

 not disappear, but its division immediately precedes that of 

 the yolk, so that its progeny must eventually become the 

 " embryo-cells" of the division masses.* The observations 

 of the same naturalist on Sagitta, and of Professor Huxley 

 on Pyrosoma, also tend to show that the '' embryo-cells" 

 are the lineal descendants of the germinal vesicle. 



The division of the " embryo-cell" immediately precedes 

 that of the yolk. The segmentation is commonly effected 

 by each division of the '' embryo -cell" becoming coated over 

 with a corresponding portion of the gTanular matter of the 

 yolk, so that every one of the multitude of minute sphe- 

 rules, into which the latter is ultimately resolved, contains, 

 as its nucleus, a derivative of the "embryo-cell." But in a 

 few cases, confined, so far as is yet known, to some Nema- 

 toid and Cestoid worms, the progeny of the original " em- 

 bryo-cell" do not coat themselves in this way, but seem 

 rather to absorb the granular and fluid matter, growing, 

 as it were, at its expense, so that the yolk entirely dis- 

 appears by the time the segmentation is complete. Pro- 

 fessor Huxley considers that a somewhat analogous pro- 



* Huxley's Oceanic Hydrozoa, p. 22 



