624 MAMMALIAN EMBRYOLOGY 



a second or false amniotic cavity may develop. Figure 357 will make 

 this clear. 



In those cases, however, where the embryo pushes through the 

 trophoblast and comes to lie as a disc upon its surface, the amnion is 

 formed quite similarly to that already described in our study of the chick. 



The region in which the embryo develops is known as the embryonic 

 shield. The primitive head-node lies about in the middle of the em- 

 bryonic shield. The primitive streak and the primitive grooves form 

 quite as in the chick, and as there stated, all structures lying anterior to 

 the head-node lie in the head proper. 



A definite notochord also forms, and the neurenteric canal can be 

 seen quite plainly at the posterior limits of the embryonic rudiment. 



There are scarcely more than half a dozen human embryos which 

 have ever been seen prior to the time of the formation of the medullary 

 plate. Then, too, none of these were of the same size, so we do not even 

 have a basis for valid comparison, and consequently, we are unable to 

 judge as to whether any of these were normal as to size and form. 



Mesoderm is formed in the mammal as it is in the chick, each meso- 

 dermal somite dividing into a somatic and a splanchnic layer. A layer 

 of entoderm joins with the splanchnic mesoderm to form the yolk-sac, 

 although no yolk is present. The trophoblast joins with the somatic 

 mesoderm to form the chorion. 



Here we may note that the term "ovum" is used in mammalian de- 

 velopment to designate any early stage in the embryo, even to the in- 

 clusion of the entire blastodermic vesicle. The term "embryo" in man 

 is given the embryo only during the first two months of its existence, 

 thereafter (that is, when the face and body are quite well formed) it is 

 known as a "foetus." 



The smallest human embryo yet seen was 1.54 mm. in length, while 

 the entire blastoderm was about 1 cm. in diameter. 



IMPLANTATION 



There are three methods by which the blastoderm attaches itself to 

 the walls of the uterus. 



First, by what is called central implantation. This occurs in the 

 ungulates and carnivores as well as in the lower primates and in some 

 rodents, such as the rabbit. In these the blastoderm becomes super- 

 ficially attached to the uterine wall, and, consequently, projects freely 

 into the lumen of the uterus. 



Second, eccentric implantation. This type is found in the mouse 

 and in some insectivora. In these forms the blastodermic vesicle lies 

 in a furrow or groove in the uterine wall. This groove is then closed up 

 so that the vesicle comes to lie in the walls of the uterus. 



Third, Interstitial implantation. In this type the blastodermic ves- 



