THE DEVELOPMENT OF THE MOLE. 147 



pit constitute the anterior fold of the amnion, and are formed 

 solely of epiblast and hypoblast. This portion of the amnion 

 does not come in contact with the wall of the uterus. 



The relations of these parts have recently been very fully 

 described by Beneden and Julin in Rabbit and Bat embryos 

 (No. 2). These authors have named this anterior fold of the 

 amnion the " pro-amnion," and have most ingeniously, and as 

 it appears to me correctly, compared it with the internal stalk 

 of the " trager ^^ of inverted types of mammalian embryos. 



I should mention that the mesoblast present in the median 

 line in the longitudinal section of an embryo of Stage e 

 (fig. 11) is concerned in the production of the heart, the 

 anterior fold of the amnion having its origin in front of 

 this mesoblast (compare figs. 11 and 34). 



The Allantois. — The allantois is, in an embryo of Stage f, a 

 short wide diverticulum of the hypoblast projecting into the 

 posterior portion of the primitive streak mesoblast behind the 

 point where the epiblast and mesoblast curve over to form the 

 amnion, and therefore also behind the point where the anal 

 pit is forming. 



This diverticulum increases in size during Stages g, h, and j, 

 and forms at the latter stage a very considerable vesicular 

 cavity opening by a narrow neck into the (future hind-gut) 

 yolk-sac beneath. The hypoblast diverticulum is formed of 

 rounded cells, and is surrounded by a mass of mesoblast through 

 which blood-vessels already ramify. The relation of these 

 parts is shown in figs. 35 and 50. 



The Vascular System. — In the earliest embryo I have examined 

 of Stage E, viz. one with only a single protovertebra, the posi- 

 tion of the heart is already indicated, and vessels are already 

 formed in the splanchnic mesoblast of the blastoderm outside 

 the embryonic area. Blood-corpuscles are, moreover, to be 

 seen within these vessels even at this early age. 



At the close of Stage f, the rudiments of the dorsal aorta 

 are present, lying some distance on each side the uotochord 

 and extending from a point on a level with the front end of 

 the heart backwards to the last protovertebra (fig. 17). They 



