EMBRYOLOGY 



147 



the course of their development having polar and equatorial 

 arrangements. These we cannot further allude to, hut may 

 mention that the extrusion of the directive bodies is only 

 temporary, they being again included within the periphery of the 

 egg by the growth and extension of adjacent parts which meet 

 over and thus enclose the bodies. 



The arrangements and movements we have briefly alluded to 

 have been limited to the unfertilised condition of the egg (we 

 should rather say, the fertilising element has taken no part in 

 them), and have as their result the union of the chromosomes 

 existing after the extrusion of the two polar bodies, into a small 

 body called the female pronucleus or egg-nucleus (Eikern), while 

 the position of the movements has been an extremely minute 

 portion of the egg near to its outer surface or periphery. The 

 introduction of a sperm, or male, element to the egg through 

 the micropyle gives rise to the formation of another minute body 

 placed more in the interior of the egg, and called the sperm- 

 nucleus. The egg -nucleus, travelling more into the interior of 

 the egg, meets the sperm-nucleus ; the two amalgamate, forming 

 a nucleus or body that goes through a series of changes resulting 

 in its division into two daughter-bodies. These two again divide, 

 and by repetitions of such division a large number of nuclei 

 are formed which become arranged in a continuous manner so 

 as to form an envelope enclosing a considerable part (if not 

 quite the whole) of the egg-mass. This envelope is called the 

 blastoderm, and together with its contents will form the embryo. 

 "VYe must merely allude to the fact that it has been considered 

 that some of the nuclei forming the blastoderm arise directly 

 from the egg-mass by a process of amalgamation, and if this 

 prove to be correct it may be admitted that some portions of 

 the embryo are not entirely the result of division or segmentation 

 of combined germ and sperm-nuclei. Wheeler states ^ that some 

 of the nuclei formed by the first differentiation go to form the 

 vitellophags scattered throughout the yolk. We should also 

 remark that, according to Henking, the blastoderm when com- 

 pleted shows at one part a thickening, immediately under 

 which {i.e. included in the area the blastoderm encloses) are the 

 two polar bodies, which, as we have seen, were formed by the 

 germinating body at an earlier stage of its activity. Fig. 79 



' J. Morphol. viii. 1893, p. 81 ; see also Graber's table on p. 149. 



