DEVELOPMENT OF CTPRINODONTS. 905 



■with blood from a median vascular tmok which divides and subdivides as it traverses 

 the ovary lengthwise, in a manner similar to that of the stem to which grapes in the 

 bnnch 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 Grsefian follicle, 

 ovisac, ovarian capsule, memirana granulosa of Von Baer, or memirana celluloaa of Coste. 

 As the egg is matured there is a space developed about it which is said to result from the 

 breaking ap of the granular layer of cells covering it. This space is fiUed with flaid, 

 and in this liquid, which increases in quantity as development proceeds, the embryo of 

 Zygonectes or top-minnow, is constantly bathed. There M no trace xohatever 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 spermatozoon. This fact 

 raises the question whether the egg membrane or zona radiaia usually present in the ova 

 of water-spawning fishes is not entirely absent in all the viviparous species. Whether 

 Batbke 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 cautions 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 question why such a unique con- 

 dition of affairs should exist in this case ? The answer, it would appear to us, is not far 

 to seek. la 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 ch^dren 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 np Internally of flat, polygonal 

 cells of pavement epithelium, and externally of a network of multipolar, fibrous, con- 



