DEVELOPMENT OF THE EMBRYO 



441 



its fringe-like margin. This region is thin and muscular and lined with 

 cilia. The oviduct proper into which the ostium leads is known as the 

 convoluted glandular portion, which is followed by a short, third portion 

 called the isthmus. It is after passing through the isthmus that the egg 

 enters the so-called uterus, which is merely a dilated portion of the 

 glandular tube. The uterus in turn opens into a short terminal region, 

 a rather thin- walled vagina, and this again opens into the cloaca, just 

 dorsal to the opening of the rectum. 



It is easier for the student to understand a developmental history 

 of the egg if it be thought of as passing through three periods. First, 

 from the beginning of the development of the ovum to the time of ovula- 

 tion. Second, from the time of ovulation through the period of fertiliza- 

 tion, and third, from the beginning of cleavage to the time the egg is 

 laid. 



First Period. (From the beginning of the development of the egg 

 to the time of ovulation.) Most animals produce a large number of eggs 

 Avithin a very brief period, while in the hen there is a long period of egg 

 formation and laying, which extends over several months, after which 

 there is a period of almost complete cessation. Undoubtedly the reason 

 for this is that when an egg becomes as large as that of the hen, which 



requires so much food in its 

 making, it is a considerable 

 drain upon the animal, and 

 secondarily, there isn't room 

 enough in the body of an 

 animal no larger than a hen 

 for many eggs of such size. 

 However, the fact that the 

 hen's ova develop in the way 

 they do, makes it possible 

 for us to observe almost a 

 complete succession of de- 

 velopmental changes from 

 the minute forms up to the 

 fully developed egg. 



In our course of general 

 biology, we learned that very early in an organism, especially in triplo- 

 blastic forms, the germplasm and somatoplasm differentiate. A few cells 

 are set aside in the innermost portion of the body of the growing embryo 

 for reproductive purposes. The development in the growing embryo of 

 the germplasm is called oogenesis in the female, and spermatogenesis 

 in the male. It will also be remembered that all the eggs are already 

 present in the ovary at the time the female is born. 



Ovum 



follitular 

 epithelium. 



Fig. 253. 



Section from ovary of adult dog. The more or less 

 star-shaped figure on the right is a collapsed follicle 

 with its contents. Below and at the right are seen the 

 tubules of the Parovarium. (After Waldeyer.) 



