122 



FUNDAMENTALS OF CYTOLOGY 



sperms, the basal portion of the tail is more or less distinctly differenti- 

 ated as a middle piece containing the centrioles and chondriosomal ele- 

 ments in a sheath of undifferentiated cytoplasm (Fig. 90) . Posterior to 

 this is a principal piece, with a thin sheath but no undifferentiated 

 cytoplasm, and an end piece, which represents the naked end of the axial 

 filament. 



The spermatozoa of many animals reveal in their external form little 

 or no evidence of the above differentiations, but taper so gradually toward 

 one or both ends that no subdivision into parts is possible on such a basis. 

 Furthermore, some animals, e.g., certain crustaceans and spiders, have 

 spermatozoa with no tails. Except for the lack of development of a motor 

 apparatus, the changes within the spermatid in such instances are funda- 

 mentally similar to those in spermiogenesis of the ordinary type. 



Fig. 91. — Maturing egg of a worm (Cerebratulus) . 1, anaphase of first meiotic 

 with first polar body budding off. 2, polar bodies completed; egg nucleus near 



division 

 center. 



Oogenesis. — In the ovarj^ the oogonia enlarge somewhat and become 

 primary oocytes. It is in these cells that meiosis is initiated. Enlarge- 

 ment continues, especially during the remarkable "growth period" which 

 comes at about the time the chromosomes are in the late pachytene and 

 the diplotene stages of the first meiotic prophase (page 111). During the 

 growth period the oocyte becomes supplied with nutritive materials 

 (yolk) and develops other features characterizing the egg. In some 

 animals, groups of nurse cells or a surrounding layer of cells, the follicular 

 epithelium, have a part in these activities. 



At the close of the growth period the chromosomes assume the compact 

 form characteristic of diakinesis and lie scattered in the enormous oocyte 

 nucleus (the germinal vesicle) . The relatively small first-division spindle 

 assumes a position perpendicular to the cell membrane, and at anaphase 

 and telophase the first polocyte, or polar body, is budded off with one of 

 the daughter nuclei (Figs. 91, 85). The nucleus remaining in the second- 



