XVI. VERTEBRATE DEVELOPMENT 



A. Development of the Frog 



The Frog's egg when observed with the naked eye appears as a 

 tiny sphere composed of a dark-colored portion, termed the animal 

 pole, and a light-colored portion, termed the vegetal pole. 

 Each egg is enclosed in a transparent vitelline membrane around 

 which are the layers of jelly. A microscopic study of prepared 

 sections shows that the protoplasm of the egg is largely concen- 

 trated in the animal pole. This fact is emphasized by the unusual 

 position of the nucleus which lies approximately in the center of the 

 animal pole. The vegetal pole contains a large proportion of non- 

 living food material, or yolk, for the use of the embryo during the 

 early stages of development before it is able to take in food from 

 the external world. (W. pp. 238-267.) 



1. Fertilization 



The sperm and the eggs of the Frog are both discharged directly 

 into the water. The former by their active swimming movements 

 reach the eggs and fertilization takes place. It is probable 

 that the sperm are attracted by certain chemical substances given 

 off by the eggs. In fertilization the nucleus of the egg and the 

 nucleus of the sperm unite in the egg cell to form the synkaryon 

 just as has been noted previously in the Invertebrates. If the 

 eggs are not fertilized no development takes place, and disintegra- 

 tion begins after they have been in the water for a while. When an 

 egg has been fertilized the synkaryon prepares at once for mitotic 

 cell division. A typical spindle is formed, and the egg soon divides 

 into equal halves which remain enclosed within the vitelline 



membrane. 



2. Early Development 



Cleavage. In general, the animal pole of the Frog's egg divides 

 more rapidly than the vegetal pole. The latter is retarded by the 

 large proportion of inert food substances, or yolk, which are pres- 

 ent in it. The early cleavages generally occur in a regular order 

 which may be described as follows. 



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