IMPREGNATION CLEAVAGE OF THE YELK. 



86 1 



the os of the one into the other uterus, a condition which is spoken of as " internal 

 migration." 



439. IMPREGNATION CLEAVAGE LAYERS OF THE EMBRYO.- 



Maturation of the Ovum. In birds and mammals, important changes occur in 

 the ovum before impregnation. The germinal vesicle comes to the surface and dis- 

 appears from view, while the germinal spot also disappears. In place of the 

 germinal vesicle, a spindle-shaped body appears. The granular elements of the 

 protoplasmic vitellus arrange themselves around each of the two poles of the spindle, 

 in the form of a star, the double-star, or diaster of Fol nuclear spindle (figs. 655, 

 656). When this takes place, the peripheral pole of the nucleus or altered ger- 

 minal vesicle, along with some of the cellular substance of the ovum, protrudes 

 upon the surface of the vitellus, where they are nipped off from the ovum in the 

 form of small corpuscles just like an excretory product (fig. 657). These bodies, 

 which are not made use of in the further development and growth of the ovum, 

 are called polar or directing globules (Fol, Biltschli, 0. Hertivig), although the 

 elimination of small bodies from the yelk was known to Dumortier [1837], Bis- 

 choff, P. J. van Beneden, Fritz Muller [1848], Rathke, and others. The remaining 

 part of the germinal vesicle stays within the vitellus and travels back towards the 

 centre of the ovum, to form the female proneucleus (0. Hertivig, Fol, Selenka, E. 

 van Beneden). [Before, however, the altered germinal vesicle travels downwards 

 again into the substance of the ovum, it divides again as before, and from it is 

 given off the second polar globule, and then the remainder of the germinal vesicle 



Fig. 655. Fig. 656. 



Fig.- 655. Formation of polar globules in a star-fish (Asterias glacialis). A, ripe ovum with 

 excentric germinal vesicle and spot ; B-E, gradual metamorphosis of germinal vesicle 

 and spot, as seen in the living egg, into two asters ; F, formation of first polar globule, and 

 withdrawal of the remaining part of the nuclear spindle within the ovum ; G, surface 

 view of living ovum with view of first polar globule ; H, formation of second polar 

 globule ; 1, a later stage, showing the remaining internal part of the spindle in the form 

 of two clear vesicles ; K, ovum with two polar globules and radial stria? around the female 

 pronucleus ; L, extrusion of polar globule. (Geddes : A-K, after Fol; L, after 0. Herlwig.) 

 Fig. 656. Egg of Scorpaena scrofa. The germinal vesicle is extruding a polar globule, 

 and withdrawing towards the centre of the ovum. Near it is the male pronucleus. 



forms the female proneucleus (fig. 655). At the same time the vitellus shrinks 

 somewhat within the vitelline membrane.] 



Impregnation. As a rule, only one spermatozoon penetrates the ovum, and as 

 it does so, it moves towards the female pronucleus, while its head becomes sur- 

 rounded with a star ; it then loses its head and cilium, or tail, the latter only 

 serving as a motor organ, w T hile the remaining middle piece swells up to form a 

 second new nucleus, the male pronucleus (Fol, Selenka). According to Flemming, 

 it is the anterior part of the head, and according to Rein and Eberth, it is the 

 head which is so changed. Thereafter, the male and female pronucleus unite, 

 undergoing amoeboid movements at the same time, to form the new nucleus of the 

 fertilised ovum. The female pronucleus receives the male pronucleus in a little 

 depression on its surface. Thereafter the yelk assumes a radiate appearance (Rein). 



