298 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



ment of the Spanish mackerel, in which it was assumed to be derived from 

 the germinal pellicle, simplifies our theory of the constitution of the tel- 

 eostean ovum. But I find myself unable to clearly determine its pres- 

 ence, as understood by Van Bambeke, in some forms, as iu Belone and 

 Alosa, for example. This layer may retain in it some part of the origi- 

 nal nuclear matter of the egg, which may be the effective agent in re- 

 ducing and effecting the incorporation of the substance of the vitellus 

 by the formation of free nuclei ffom part of the original nuclear sub- 

 stance which has remained in the intermediary layer, which is immedi- 

 ately in contact with the yelk. But I have shown good reasons, as they 

 have appeared to me, for regarding the intermediary layer as really 

 equivalent to the hypoblast. If this view be sustained, and na evidence 

 to the contrary derived from sections made during the early stages has 

 yet been brought to light, either by the researches of myself or others, 

 it would appear that we may rightfully maintain that the blastoderm of 

 the fish is the homologue of the whole of the amphibian or marsipobranch 

 ovum, and that the yelk has been superadded and is not directly con- 

 cerned in the process of development, at least not until about the time 

 the tail of the embryo begins to be budded out, shortly after which the 

 heart is developed and begins to pulsate. The migration of the nucleus 

 of the teleostean egg, towards the surface and apparently into the per- 

 ipheral germinal matter is, I apprehend, a very different thing from 

 what occurs in the ova of the lamprey and frog, though upon comparison 

 they present a superficial resemblance. The behavior of the ovum of 

 the sturgeon, according to Salensky, appears to be similar to that of the 

 teleost in respect to the formation of the germinal disk; the nucleus, too, 

 seems to undergo disintegration into fragments. 



Summari2;ing the arguments presented in the foregoing pages the 

 following conclusions appear to me to be warranted: 



1. The germinal disk of the teleostean &gg is homologous with the 

 whole of the am[)hibian and marsipobranch ovum. 



2. The yelk, while it is in intimate organic union with the blastoderm, 

 may be regarded merely as a nutritive appendage to the teleostean egg 

 from the center of which the nucleus has migrated at about the end of 

 intraovarian development into the germinal pellicle or disk, leaving the 

 yelk a passive structure, the presence of which has greatly modified the 

 mode of development of the blastoderm. 



3. The rim of the blastoderm is more or less extensively transformed 

 into the body of the embryo as argued by His and Eauber. 



4. The difference between the development of the ganoids and teleosts 

 is much less than between the former and amphibians. 



5. The blood in Belone is developed directly from the yelk through 

 the intermediation of the hypoblast, quantities of its corpuscles being 

 found in the heart or pericardiac chamber. 



6. The intestine of the teleost embryo is formed from behind forwards 

 by splitting of the hypoblast, and not by ah invagination conterminous 



