VERTEBRATE DEVELOPMENT 237 



the young are born in a comparatively helpless and immature 

 condition. (W. f. 134, a, d, g.) 



Except in the Prototheria, the mammalian egg, quite unlike 

 that of the Frog and Hen, is very small, being in general micro- 

 scopic in size, and containing practically no stored food for the 

 nourishment of the embryo during the early stages of develop- 

 ment. It is therefore necessary, in order for development to pro- 

 ceed, that the fertilized egg receive nourishment from the mater- 

 nal tissues very soon after development begins. This is supplied as 

 described below. (W. f. 133.) 



If sperm are present the egg is fertilized soon after it enters the 

 oviduct, and begins to divide. The cleavage is holoblastic and 

 soon results in the formation of a spherical body of cells, known as 

 a morula, which may be regarded as similar to the blastula stage. 

 Except for the complete cleavage, the early development of the 

 mammalian egg, including the formation of the various organs, 

 shows great similarity to that of the Chick. A particularly note- 

 worthy feature of the mammalian embryo at this stage is the very 

 early formation of the primary germ layers and the emrryonic 

 memrranes such as the amnion. A study of the internal structure 

 of the embryo at the morula stage shows that the cells are differ- 

 entiated into an inner cell mass, which contains the rudiments 

 of the embryonic body proper, and an outer layer, known as the 

 trophorlast, which develops from the ectoderm at the surface and 

 encloses the entire embryo. (W. f. 176.) 



During the cleavage stages the embryo is passing through the 

 oviducts down into the uterus, where it is to be attached and re- 

 tained for a time during development. By the time it reaches the 

 uterus the outer covering of trophoblast has been formed. This 

 layer is very essential to the developing embryo, for the cells of 

 which it is composed are able to secrete a ferment which erodes 

 the small portion of maternal tissue lining the uterus with which 

 the tiny embryo is in contact. Thus the embryo attaches itself 

 to the mother, becoming actually embedded in the uterine wall. 

 Nutritive materials are temporarily secured by the digestive and 

 absorptive action of the trophoblast. A little later, by a combina- 

 tion of trophoblastic tissues of the embryo with maternal tissues 

 in the uterine wall, a very remarkable mammalian structure, 

 known as the placenta, is formed which connects the embryo with 

 the maternal tissues and through which it is nourished. Leading 



