THE OVUM. 



1151 



hence the fully developed spermatozoon contains only one-fourth of the nucleus 

 of the original spermatocyte. The matured ovum and the spermatozoon may 

 therefore be looked upon as of the same morphological value. 



FIG. 689. Formation of polar bodies in Asterias glacialis. (Hertwig.) In i. the polar spindle (p) has 

 advanced to the surface of the egg. In n. a small elevation (rk) is formed which receives a half of the 

 spindle. In in. the elevation is constricted off, forming a polar body (rk'). Out of the remaining half ot 

 the previous spindle a second complete spindle (sp) has arisen. In iv. is seen a second elevation, which in 

 v. has become constricted off as the second polar body. Out of the remainder of the spindle (iv.) is developed 

 the female pronucleus. 



FERTILIZATION AND SEGMENTATION OF THE OVUM. 



The first changes in the ovum which take place at the time of conception 

 are as follows : 



1. Impregnation. One, or perhaps more, spermatozoa penetrate the zona 

 pellucida and are contained in the perivitelline fluid. A single spermatozoon, 



f.vr 



FIG. 690. Fertilization of the ovum of an echinoderm. s. Spermatozoon, m. pr. Male pronucleus. f.pr. 

 Female pronucleus. 1. Accession of a spermatozoon to the periphery of the vitellus. 2. Its penetration. ! 

 Transformation of the head of the spermatozoon into the male pronucleus. 4, 5. Blending of the male and 

 female pronuclei. (From Quain's Anatomy, Selenka.) 



more advanced than the rest, becomes buried in the yolk, the tail disappears, 

 and the head constitutes the male pronucleus. This gradually approaches the 

 female pronucleus, and ultimately the two pronuclei come into contact and fuse 

 to form a new nucleus, containing both male and female elements, and named 

 the segmentation or cleavage nucleus, and the whole cell thus modified is called 

 the blastosphere (Fig. 690). It seems as if this normally occurs in the Fallopian 

 tube, 1 but it is possible that it sometimes takes place before the ovum has entered 



1 Many physiologists, as Bischoff and Dr. M. Barry, taught that the ovum is fecundated in the 

 ovary, but the reasoning of Dr. Allen Thomson appears very cogent in proving that the usual spot 

 at which the spermatozoa meet the ovum is in the tube, down which it slowly travels to the uterus, 

 in its course becoming surrounded by an albuminous envelope derived from the walls of the tube. 



