66 CLEAVING OF THE YOLK 



vertebrata, before noticing the corresponding changes which have been 

 observed to take place in the ova of Mammalia. 



In the Invertebrata. From the investigations of the former of these 

 observers, it would appear, that the early structural changes undergone by 

 the substance of the impregnated ovum, are of several varieties in the 

 different invertebrate animals ; but that, so far as has yet been ascertained, 

 these varieties are referable to three principal types. In the first type, the 

 whole substance of the yolk undergoes the process of division and subdivi- 

 sion. In the second, the process is confined to a portion of the yolk. And, 

 in the third type, the yolk does not take any part in the process, but 

 certain transparent nucleated cells, arising in its interior, undergo multi- 

 plication in the same manner as the yolk itself in the former varieties : 

 the substance of the yolk in this third type being gradually absorbed 

 during the process. 



I. The first of these types is exemplified in the ova of three species of 

 Ascarides, namely, Ascaris nigrovenosa, A. acuminata, and A. succisa.* 

 Kblliker f states, that as soon as the mature ova of one of these worms 

 reach the fundus of the uterus, the first signs of impregnation are mani- 

 fested by the disappearance of the germinal vesicle, and a diminution in the 

 consistence of the yolk, the granules of which adhere together less firmly 

 than before. Shortly after this change, a new nucleated cell appears in the 

 centre of the yolk, which then again acquires a closer texture, a smaller 

 circumference, and a more definite outline. After a time, two cells in- 

 stead of one are perceived in the interior of the yolk, and soon the yolk 



itself divides into two halves, each of which contains one of the cells 

 in its centre (fig. 6, A). Then, again, each of the two cells is replaced 

 by two others, and the substance of the yolk becomes divided into four 



* The development of these Entozoa was described by Von Siebold (Burdach's Physiolo- 

 gic. Second edition, vol. ii.), but has been more recently and more accurately inves- 

 tigated by Bagge and Kblliker in the essays above alluded to. 



t Muller's Archiv. 1843, p. 103. 



Fig. 6. Cleaving of the yolk after fecundation. A. An ovum, the yolk of which is 

 divided into two equal portions ; the upper portion contains a cell with a large nucleus, the 

 lower, a similar cell with two small nuclei. B. An ovum, of which the yolk is divided into 

 four masses, three of which possess a single nucleated cell, the fourth, two such cells. 

 C. An ovum, the globular masses of whose yolk amount to sixteen, in each of which a 



