DEVELOPMENT OF CYPRINODONTS. 905 
with blood from a median vascular trunk which divides and subdivides as it traverses 
the ovary lengthwise, in a manner similar to that of the stem to which grapes in the 
bunch are attached. In this way it happens that each egg or ovum has its own inde- 
pendent supply of blood from the general vascular system of the mother, from which the 
material for the growth and maturation of the egg is derived, and which afterward 
becomes specialized into a contrivance by which the life of the developing embryo 
is maintained while undergoing development in their respective follicles in the ovary 
or egg-bag. The ova develop along the course of the main. vessel and its branches, as 
may be learned upon examining a hardened specimen, where the very immature ovarian 
eggs are seen to be involved in a meshwork of connective fibrous tissue, which serves 
not only to strengthen the vessels but also afterwards enters into the structure of the 
walls of the ovarian sacs or follicles externally. 
*‘The very immature eggs measure from less than a hundredth of an inch up to a 
fiftieth, and on up to a twelfth of an inch, when they may be said to be mature. They 
develop along a nearly median rachis or stalk which extends backward and slightly 
downward and which gets its blood supply very far forward from the dorsal aorta, The 
ova, after developing a little way, are each inclosed in a follicle, the Grzefian follicle, 
Ovisac, Ovarian capsule, membrana granulosa of Von Baer, or membrana cellulosa 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 of 
Zygonectes or 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 mycropolar openings or pores for the entrance of the spermatczoon. 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 account 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. Suffice it to say, however, that with very cautious preparation, staining and 
dissection of the follicles inclosing the ova of Zygonectes, I have completely failed to dis- 
cover what I could regard as an egg membrane, although personally familar with the 
appearance 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 Mueller. No such structure existing as a covering 
for the egg of Zygonectes, we are in a position to ask the qnestion why such a unique con- 
dition of affairs skould 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 supplied with a covering more or less firm and capable of protecting the con- 
tained embryo, which in the case of the top-minnow is not needed, because the embryo is 
developed so as to be quite competent to take care of itself as a very well organized lit- 
tle fish when it leaves the body of its parent. Nature will not waste her powers in an 
effort to make useless clothes for such of her children as do not need them; on the con- 
trary, she is constantly utilizing structures economically, and often so as to serve more 
than one purpose, This is the apparent answer to the query with which we started. 
‘‘ The follicles or sacs containing the ova are built up internally of flat, polygonal 
cells of pavement epithelium, and externally of a network of multipolar, fibrous, con- 
