1 8 8 2 .] Incubation of the Top-Min now ( Gambusia) . 1 1 1 



membrana granulosa of Von Baer, or membrana celhUosa of Coste. 

 As the egg is matured there is a space developed about it which 

 is said to result from the breaking up of the granular layer of 

 cells covering it. This space is filled with fluid, and in this liquid, 

 which increases in quantity as development proceeds, the embryo 

 top-minnow is constantly bathed. There is no trace whatever 

 in the egg of this fish of an independent egg membrane, as is 

 the case with all known forms which spawn directly into the 

 water, and which is usually, if not in all cases, perforated by 

 one or more micropylar openings or pores for the entrance of 

 the spermatozoon. This fact raises the question whether the egg 

 membrane or zona radiata usually present in the ova of water- 

 spawning fishes is not entirely absent in all the viviparous species. 

 Whether Rathke has recorded anything on this point in his ac- 

 count of the development of Zoarces, the viviparous blenny, I am 

 not able to say at present, as I do not have access to his memoir. 1 

 Suffice it to say, however, that with very cautious preparation, 

 staining and dissection of the follicles inclosing the ova of Gam- 

 busia, I have completely failed to discover what I could regard 

 as an egg membrane, although personally familiar with the ap- 

 pearance of the coverings of the ova of more than twenty species, 

 embracing fifteen or more families. The zona radiata or covering 

 of the egg in other bony fishes is said to be secreted from the 

 cells lining the follicles and is composed of a gelatinoid substance, 

 and it is often perforated all over by a vast number of extremely 

 fine tubules, called pore canals by their discoverer, Johannes Muel- 

 ler. No such structure existing as a covering for the egg of Gam- 

 busia, we are in a position to ask the question why such an unique 

 condition of affairs should exist in this case ? The answer, it 

 would appear to us, is not far to seek. In the case of eggs which 

 ordinarily hatch in water it is necessary that they should be sup- 

 plied with a covering more or less firm and capable of protecting 

 the contained embryo, which in the case of the top-minnow is not 

 needed, because the embryo is developed so as to be quite com- 

 petent to take care of itself as a very well organized little fish 



