382 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



These are (1) " giant" spermatids, resulting from mitosis in which 

 neither nucleus nor cell body has divided ; (2) " multiple " spermatids, 

 resulting from divisions in which the nuclei alone have separated. 



These plurivalent spermatids have from one to four centrosomes accord- 

 ing as they correspond to from one to four normal spermatids. There is 

 a tail filament in connection with each centrosome irrespective of the 

 numerical conditions of nuclei or of archoplasmic masses. 



Oogenesis. — Oogonia (Figs. 109-119). — The division of the oogonia 

 is of the ordinary somatic type, the chromosomes being formed by the 

 direct condensation of the chromatin segments without any intermediate 

 formation of chromomeres. 



Oocytes (Figs. 120-138). — The young oocytes undergo a series of 

 prophase changes which do not lead to nuclear division. The nucleolar 

 shell breaks down and contributes to the chromatin reticulum. The chro- 

 matin condenses into separate " beaded " segments formed by the union 

 of varying numbers of karyosomes. The central portion of the nucle- 

 olus persists, but, unlike the corresponding plasmosome of the sper- 

 matogonia, it combines with basic instead of acid dyes. 



The chromatin segments then become irregular and break down, and 

 the nucleus, instead of undergoing division, returns to the "resting" 

 condition, in which the chromatin is diffused. The central portion of 

 the nucleolus persists, and develops into the chief nucleolus of the 

 growth period of the oocyte. 



Growth period (Figs. 129-138). — In the early growth period the 

 chromatin largely loses its staining capacity. Later the chromatin re- 

 asserts itself and appears in block-like masses of granules. These blocks 

 arrange themselves in strands, which, though they stain deeply, with 

 haematoxylin, select acid dyes ; they frequently form Y- or V-shaped 

 figures. 



The chief nucleolus grows, becomes vacuolated, and frequently shows 

 strands and granules in its ground substance. It takes no part in the 

 formation of the chromatin strands. Throughout its existence it selects 

 basic dyes. 



During the early growth period accessory nucleoli are formed. These 

 arise in connection with the nuclear reticulum ; they are irregular in 

 outline, and agree in staining reactions with the, chromatin strands. 

 During the later growth period they increase in size. These nucleoli 

 select acid dyes. 



Fertilization (Figs. 139-149). — The sperm structure after penetra- 

 tion consists of head and middle piece ; it is surrounded by astral radia- 



