10 INTRODUCTION. 



takes place when an egg has reached its full size, irrespective of 

 any influence, direct or indirect, on the part of the male animal ; 

 and it occurs in the same manner whether the egg is going to 

 develop into an embryo or not. Apparent exceptions to this 

 statement are met with in cases such as the lamprey and the 

 frog, in which the first polar body is extruded prior to entrance 

 of the male element or spermatozoon, but the second one during 

 or after that process ; while in some few instances, as in Ascaris 

 according to Van Beneden, both polar bodies are extruded 

 after the entrance of the spermatozoon into the egg. It is 

 probable that in these cases the polar bodies are formed as usual, 

 before the entrance of the spermatozoon, but are not extruded 

 from the egg until after that event. 



2. The changes in the egg-nucleus that precede or accom- 

 pany the formation of polar bodies are of two distinct kinds : 

 (i) The preliminary reduction in size of the nucleus, and 

 diffusion of the greater part of its substance through the pro- 

 toplasm of the egg ; (ii) the division of the small remaining 

 portion of the nucleus into the female pronucleus and the 

 polar bodies. These two processes are clearly of widely different 

 nature. 



3. Of the former of these processes the following explanation 

 has been offered. The great size of the egg-nucleus distinguishes 

 it from the nucleus of almost all other cells, excepting nerve 

 ganglion cells, and is probably associated with the large size of 

 the egg itself and the nutritive changes necessary for its forma- 

 tion and elaboration. When the egg has attained its full size 

 the nutritive or trophic function of the nucleus is fulfilled, and 

 the portion of the nucleus concerned with these processes 

 becomes merged in the protoplasm of the egg-body. 



4. In the formation of the polar bodies, the nuclear changes 

 that precede or accompany the process appear to be the same 

 as those which occur during the division, by mitosis, of an ordi- 

 nary epithelial cell. At each division to form a polar body the 

 chromatin threads of the nucleus appear to be halved precisely ; 

 so that the female pronucleus contains exactly one-fourth of 

 the quantity of chromatin present in the nuclear spindle of the 

 egg-nucleus. 



5. Authorities differ as to whether any part of the proto- 

 plasm of the egg is extruded with the daughter nuclei in the 



