DEVELOPMENT. 7(57 



in his own words: " My view amounts to the following, viz., that after 

 the formation of the polar-cells, the remainder of the germinal vesicle 

 within the ovum (the female pro-nucleus) is incapable of further devel- 

 opment without the addition of the nuclear part of the male element 

 (spermatozoon), and that if polar-cells were not formed, parthenogenesis 

 might normally occur." 



Changes following Impregnation. The process of impregnation 

 of the ovum has been observed most accurately in the lower types. In 

 mammalia, although spermatozoa pass in numbers through the yolk 

 envelope, yet their further progress is only inferred from observations on 

 the lower animals. The process in asterias glacialis, according to Bal- 

 four, is as follows: The head of a single spermatozoon joins with an 

 elevation of the yolk substance, the tail remaining motionless, and then 

 disappearing. The head enveloped in the protoplasm then sinks into the 

 yolk and becomes a nucleus, from which the yolk substance is arranged 

 in radiating lines. This is the male pro-nucleus. At first, at some dis- 

 tance from the female pro-nucleus, it after a while approaches nearer, 

 and the female pro-nucleus, which was before inactive, becomes active. 

 The nuclei at last meet and unite. The result of their union is the first 

 segmentation sphere, or blasto-sphere. It is a nucleated protoplasmic cell. 

 The changes which have resulted in the formation of the blasto-sphere 

 or primitive segmentation germ are followed by the process known as 

 segmentation of the yolk. 



This process and the earlier stages in development are so fundamen- 

 tally similar in all vertebrate animals, from fishes up to man, that the 

 gaps existing in our knowledge of the process in the higher mammalia, 

 such as man, may be, in part, at any rate, filled up by the more accu- 

 rate knowledge which we possess of the development of the ovum in 

 such animals as the trout, frog, and fowl. 



One important distinction between the ova of various vertebrata should be 

 remembered. In the hen's egg, besides the shell and the white or albumen, two 

 other structures are to be distinguished the germ, often called the cicatricula 

 or " tread, " and the yolk, inclosed in its vitelline membrane. 



The germ is (as was mentioned in the description already given) essentially 

 a cell, consisting of protoplasm inclosing a nucleus and nucleolus. It alone 

 participates in the process of segmentation, the great mass of the yolk (food- 

 yolk) remaining quite unaffected by it. Since only the germ, which forms but 

 a small portion of the yolk, undergoes segmentation, the ovum is called mero- 

 blastic. 



In the mammalia, on the other hand, there is no large unsegmented mass 

 corresponding to the food-yolk of birds ; the entire ovum undergoes segmenta- 

 tion, and is hence termed holoblastic. 



The eggs of fishes, reptiles, and birds, are meroblastic, while those of am- 

 phibia and mammalia are holoblastic. 



Of the changes which the mammalian ovum undergoes previous to 



