460 



HISTOLOGY 



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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 



,1 i . T . 



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 

 known as the yolk nucleus. 

 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 



FIG. 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 



r ' 



has the appearance of either a centrosome or a 

 nebenkern or a yolk nucleus. 



In turning once more to the important structural changes which take 

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



FlG 4s- Portion of the nurse- 



cell layer of the egg-follicle 



O f a cat. ov., edge of ovum. 

 x 8 ?- 



