THE EARLY DEVELOPMENT OF THE FROG 71 



The spermatozoa of the frog are quite typical in size and 

 form (Fig. 23, A, 5). They are about 0.1 mm. in length; the 

 head is comparatively long, in some species tapered and curved 

 at the apex with a laterally attached adhesive perforatorium 

 or in others, quite blunt with a small rounded perforatorium. 

 The anterior centrosome lies in the head, the posterior in the 

 middle piece. 



B. THE FORMATION OF THE GERM CELLS 

 1. The Reproductive Organs 



Before describing the formation of the ova and spermatozoa 

 it will be necessary to recall the essential arrangement of the 

 gonads and their ducts in the mature organism. The develop- 

 ment of the system will be described later. 



The single pair of ovaries are proliferations of the cells form- 

 ing the longitudinal genital ridges. These project from the 

 body wall for some distance along either side of the dorsal 

 attachment of the mesentery (Fig. 24). Each is surrounded 

 by a peritoneal fold (mesovarium} , which also slings the organ 

 from the dorsal body wall and transmits its nervous and vas- 

 cular supplies. Each ovary is divided transversely into a 

 series of compartments. In each of these is a small internal 

 cavity the thick wall of which is formed by the germinal tissue 

 proper. After spawning the ovaries are left as small rudi- 

 ments, and during the following summer eggs are formed in 

 large numbers and their growth is practically completed be- 

 fore the beginning of the period of hibernation. The .ova jire 

 all formed from a few primitive ova which divide repeatedly, 

 forming small groups or nests of cells, one of which enlarges 

 becoming the ovum proper, while the others around it be- 

 come the nutritive follicle cells (membrana gramdosa) . As the 

 growth of the ova is completed, the ovaries are so enlarged 

 that they occupy a large part of the body cavity and crowd 

 upon the other viscera. 



Loosely attached to the anterior ends of the ovaries are the 

 fat bodies large masses of yellow streamers of lymphatic 



