CHAPTER VIIL 



EMBRYOLOGY. 



As this branch of the subject is better studied from a physiological 

 point of view, we must refer the student to some able work on 

 physiology for a description of the changes in, and development 

 of, the fecundated germ of the future animal. Our few remarks, 

 for the most part, apply to the fully, or nearly fully, developed 

 foetal animal. 



As we have previously seen, the ovum is the germ of the 

 future animal, while the fecundating principle is the spermato- 

 zoon. From the period of fecundation to that of parturition, oi 

 birth of the young, the interval is said to be that of utero-gesta- 

 Hon. This interval differs in different classes of animals, but in 

 the same class it persists with singular uniformity, the variations 

 being small. The period of utero-gestation in the mare is 48 

 weeks; in the cow, 40 ; in the sheep, 21, or 22 ; in the sow, 

 16, or 17 ; in the bitch, 9 ; and in the rabbit, about 4 weeks. 



In its early stages of development the fecundated ovum is 

 known as the embryo ; in the later stages it is called the foetus ; 

 but the exact condition at which this change of name occurs is 

 indeterminate. The foetus occupies the cavity of the uterus, 

 the neck of that organ being closed during gestation by the 

 Nabothian glands. 



OVUM AFTER FECUNDATION. 



The primary changes undergone by the fecundated ovum, 

 interesting as they are, can be merely alluded to here. By 

 repeated fusion of the vitelline substance there arises a cellular 

 material called the emhryogenic tissue, or blastoderm, out of 

 which the body of the embryo is formed. This tissue divides 

 into three layers, an outer, a middle, and an internal, called 

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