460 



HISTOLOGY 



We may conclude that the yolk passes in as a fluid, or that there 



are canals which are too fine to be detected. 



The growing ovum often, 

 but not always, possesses 

 another structure which is 

 but little understood, but 

 which appears to be con- 

 cerned with its yolk accum- 

 ulation. This is a body in 

 the cytoplasm which is 

 ~ NX ^,/. / known as the yolk nucleus. 



9eC ' T 



Its most characteristic ap- 

 pearance, perhaps, is in 

 some of the fishes, as, for 

 example, in the yolk nu- 

 cleus of Lophius, the angler, 

 where it can be seen in the 

 egg a third grown, as a 

 denser, darker-staining mass 

 in the middle cytoplasm 

 (Fig. 426). 



It has been described in 

 another characteristic form 

 by Calkins in the earth- 

 worm, Van Bambeke in a 

 In this last type it first appears 



onn. t. 



FlG. 424. Part of the multiple layer of nurse cells in 

 the egg-follicle of the half-grown ovum of a water 

 snake, Natrix sipedon. One of the four large secret- 

 ing cells (sec.c.) shows a process extending through 

 the cell wall of the egg. ov., edge of ovum; conn.t., 

 connective-tissue layer of follicle; sec.c., secreting 

 nurse cells. X 720. 



spider, and Crampton in an ascidian. 

 as an entire or partial ring of the dark-staining 

 material encircling the nucleus. This ring re- 

 treats peripherally, at the same time undergo- 

 ing a disintegration into smaller bodies or even 

 into diffused granules. It finally becomes dis- 

 tributed throughout the cytoplasm and thus is 

 lost as a visible, structural feature of the cell. 

 Figure 427 is a series of three stages as figured 

 by Van Bambeke in a spider, Pholcus. 



A. centrosome has been described as occur- 

 ring in young, resting ova as well as those seen 

 during divisions. Munson has described a pe- 

 culiar body, in the ovum of Limulus, which FIG 425. Portion of the nurse- 



cell layer of the egg-folhcle 



has the appearance of either a centrosome or a O f a cat. ov., edge of ovum, 

 nebenkern or a yolk nucleus. x 8 7- 



In turning once more to the important structural changes which take 

 place in the nucleus of the growing ovum, we find ourselves confronting 



