1 882.] Incubation of the Top-Mmnow ( (iambusia). 115 



With fluid and grows with the growth of the germinal disk, as the 

 latter becomes converted into the blastoderm, and does not disappear 

 until some time after the embryo has left the egg as a young fish ; 

 and then it often remains as a space around the yelk sac for as 

 long as a vestige of the latter remains, as may be seen in the 

 young of Cybium, Parephippus, Gadus, Elacate and Syngnathus. 

 In regard to this point, I hold views entirely different from any 

 other observers, but inasmuch as the writer has had opportuni- 

 ties for the study of the development of a greater number of spe- 

 cies, representing a greater number of families, than any previous 

 investigator, and because the observations are based on material 

 studied without the use of hardening re-agents which either 

 deform or obliterate the segmentation cavity, and also because it 

 was found to be present in all of the forms which were sufficiently 

 well studied, it is believed that it will be found in the developing 

 ova of most or all Teleostean fishes. Should this prove to be the 

 fact, the Teleostean egg will be as distinctly defined in respect to 

 the sum of the developmental characters which it presents, from 

 the developing ova of other vertebrates, as the adult Teleost is 

 from the remaining classes of the sub-kingdom to which it be- 

 longs. The floor of the cavity appears to be formed by the hypo- 

 blast or innermost embryonic layer, while its roof is formed by 

 the epiblast or outermost skin layer. Gradually this blastoderm, 

 which has been derived by cleavage from the germinal disk, 

 grows over the yelk, no part of its epiblast layer being in direct 

 contact with the hypoblast below on account of the presence of the 

 intervening film of fluid, except at its rim. The embryo is also 

 found to be in fixed contact with the yelk. The blastoderm 

 grows at about an equal rate all around its margin ; the point 

 where the edges of the blastoderm finally close is almost directly 

 opposite the site where the germinal disk first appeared ; the clo- 

 sure at last occurs just behind the tail of the embryo where a 

 little crater-like elevation marks the point at which it disappears. 

 The ernbryo now lies along a meridian of the blastoderm ; its 

 head at the original germinal pole, its tail at the other. The 

 growth of the blastoderm over the yelk is greatly facilitated by 

 the film of fluid contained in the segmentation cavity, over which 

 it can glide as it grows without friction. This view seems to me 

 to be the most rational yet proposed in explanation of the 

 method by which the blastoderm grows laterally in all directions 



