DEVELOPMENT. 657 



Balfour's view as to the formation of the polar bodies may be given 

 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." 



(2.) Changes following Impregnation. The process of impreg- 

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

 In mammalia, although spermatozoa pass in numbers through the yelk 

 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 yelk substance, the tail remaining motionless, and then 

 disappearing. The head enveloped in the protoplasm then sinks into 

 the yelk and becomes a nucleus, from which the yelk substance is 

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

 some distance 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 proto- 

 plasmic 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 yelk. 



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 accurate 

 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 cicafcricula or "tread," and the yellc, 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 nucle- 

 olus. It alone participates in the process of segmentation, the great mass 

 of the yelk (food-yelk) remaining quite unaffected by it. Since only the 

 germ, which forms but a small portion of the yelk, undergoes segmenta- 

 tion, the ovum is called meroblastic. 



In the Mammalia, on the other hand, there is no large unsegmented 

 mass corresponding to the food-yelk of birds; the entire ovum undergoes 

 segmentation, and is hence termed holoblastic. 



The eggs of Fishes, Reptiles, and Birds, are meroblastic, while those 

 of Amphibia and Mammalia are holoblastic. 

 42 



