790 



EMBRYOLOGY 



the nucleus, or germinal vesicle, contains a fine reticulum of linin fibres, 

 chromatin threads, and the nucleolus, or germinal spot. By a process 

 of karyokinesis confined to the nucleus, this divides in the same way as 



PB 



Fig. 216. Germ-nuclei of the thread-worm ascaris megalocephala (Boveri). 



A, ovum immediately after the formation of the second polar body (PB) ; , egg-nucleus; S, 

 sperm-nucleus, derived from the head of a spermatozoid that has entered the ovum. B, following 

 stage, in which , the egg-nucleus, and 6", the sperm-nucleus, have assumed the same size and struc- 

 ture. C, each nucleus has been' transformed into two chromosomes, E, S; A, attraction sphere 

 containing a centrosome; PB, polar bodies. D, karyokinetic figure; C, chromosomes splitting 

 lengthwise; A, one of the two asters; S, chromosomes forming a chromatic spindle. 



the nucleus of an ordinary tissue-cell. After division into two, one of 

 the daughter-nuclei is extruded from the ovum. This is the first polar 

 body, and it carries with it a certain number of chromosomes. Sometimes 



