^8 J. M. ESSENBERG. 







They remain on the outer periphery of the gonads, apparently in- 

 active. All activity is vested in the cells derived from the peri- 

 toneum. These cells form at first a solid cord, the sex-cord, in the 

 central part of each gonad, parallel with the longer axis of the 

 animal. The cord formation- in the young testis is antero-poste- 

 rior, but as the elongation of the gonad is mostly anteriorly, the 

 cord formation later follows the elongation of the testis. The cord 

 resembles very much a tubular gland except that there is no lumen 

 at first. The peritoneal cells place themselves side by side with 

 their apices pointing centrally in the sex-cord, which in a trans- 

 verse section appears rosette-like (Fig. 31). These cells increase 

 considerably in size and number, which causes the increase in diam- 

 eter of the cord and the origin of a central lumen ; or, in other 

 words, transforms the cord into a tube which is the testicular cavity 

 or sperm duct. It must be pointed out that there is no indication 

 whatever that any of the germ cells participate in the formation of 

 the sperm duct. 



3. Late Stage of Tubule Formation. 



The size limits of animals in this stage vary from 15.7 to 51.5 

 mm. in total length. The wide range is significant in that it shows 

 great variation in the time of sex-differentiation in the male. The 

 average form index of this period is 4.66 and the fin ratio is 1.74. 

 which is decidedly male. There is a slight increase in diameter in 

 the third ray in the preceding stage, but', strictly speaking, trans- 

 formation of the anal fin into an intromittent organ or gonopod 

 does not begin before the present stage is reached. The gonad is 

 decidedly differentiated as testis before the anal fin is affected. 

 The first indication of the transformation of the anal fin into a 

 gonopod is the thickening of the third ray. This is shortly fol- 

 lowed by the elongation of the third, fourth, and fifth rays until 

 approximately twice the length of the original fin is reached. The 

 rest of the rays i.e., the first, second, and seventh to eleventh- 

 are not subject. to any particular change. 



The gonad has increased appreciably in size owing primarily to 

 the proliferation of the peritoneal derivatives and only secondarily 

 to increase of the primordial germ cells. Most commonly the 

 testis is butterfly-shaped in the transverse section, but may be very 



