438 



HISTOLOGY 



The next stage (Fig. 397) shows spermatogonial divisions and at the 

 same time the nurse cells can be seen in the midst of their migration. 



FIG. 396. Part of a section of an older 

 sperm lobule of Raja ocellata than that 

 seen in Fig. 395. Nurse cells (n.c.) lie 

 in a distal position. Spermatogonia 

 numerous. X 1000. 



FIG. 397. Part of an older sperm lobule of 

 Raja ocellata than seen in Fig. 396. Nurse 

 cells (n.c.) migrating to a proximal posi- 

 tion. Spermatogonia dividing, x 1000. 



feliP 



In Figure 398 this migration is finished and the reproductive cells 



(spermatogonia) have just finished their multiplication divisions. At 



-.-rv^, ..& this time they are from six to 



.Vi'*v3PM*i3 * 



eight rows deep and must num- 

 ber in the neighborhood of 8000 

 spermatogonia for each lobule. 



The spermatogonia now go 

 through the synizesis stage, or 

 contraction stage, which is rep- 

 resented in Figure 398. After 

 this they expand the chromatin 

 thread and appear as the cells 

 on the left of Figure 399. They 

 are now known as the primary 

 spermatocytes. In these it can 

 be seen that the chromatin shows 

 ring-like structures at places. 

 These are the forming tetrads 

 spoken of in the general discus- 

 sion, and they soon become 

 closely assembled in a dense, 

 equatorial plate, which then di- 

 vides by a pulling of the ring- 

 shaped chromosomes into two halves. This may be seen in the right- 

 hand portion of Figure 399. 



FIG. 398. Part of an older sperm lobule of Raja 

 ocellata than seen in Fig. 397. Nurse cells now 

 all rest on proximal surface. Spermatogonia in 

 synizesis or contraction stage. X .100. 



