THE EMBRYO. 



1159 



mesenteric vessels. The latter vessels become atrophied as the functional activity 

 of the body with which they are connected ceases. 



So far we have traced : (1) the segmentation or cleavage of the ovum and the 

 formation of a blastodermic vesicle, consisting of (a) an external envelope and 

 (b) an internal mass of cells applied to it at the embryonal pole, but separated 

 elsewhere by an albuminous fluid. (2) The separation of the hypoblast from the 

 inner surface of this internal mass and its extension as a lining to the external 

 envelope. (3) The development of the epiblast, also from the internal mass of 

 cells, absorption taking place between it and the external layer to form a cavity 

 the primitive amniotic cavity. (4) The formation of an oval-shaped disk, the 

 germinal disk, and the appearance of the primitive streak at its posterior end. 



FIG. 704. Magnified view of the human embryo of four weeks, with the membranes opened. (From Leish- 

 man, after Coste.) y. The umbilical vesicle with the omphalo-mesenteric vessels, v, and its long tubular attach- 

 ment to the intestine, c. The villi of the chorion. TO. The amnion opened, u. Cul-de-sac of the allantois, and on 

 each side of this the umbilical vessels passing out to the chorion. In the embryo : a. The eye. e. The ear- 

 vesicle, h. The heart. 1. The liver, o. The upper; p, the lower limb, w. Wolffian body, in front of which are 

 the mesentery and fold of intestine. The Wolman duct and tubes are not represented. 



(5) The development of the mesoblast from the primitive streak and its extension 

 between the epiblast and mesoblast. (6) The formation of the " neural groove " 

 in front of this primitive streak, caused by the gro wing-up of the epiblast on either 

 side of it, so as to form two longitudinal ridges, called the "laminae dorsales." 

 (7) The increase and incurvation of these laminae dorsales, until they meet dorsally 

 and enclose the " neuraj canal," from the epiblastic lining of which the nervous 

 centres are developed. (8) The formation, from the hypoblast immediately 

 under the canal, of a continuous rod of cells, the " cauda dorsalis" or " noto- 

 chord." (9) The formation, from the paraxial mesoblast, on either side of the 

 notochord, of a number of square segments, the " protovertebrae " or "meso- 

 blastic somites." (10) The splitting of the lateral mesoblast into two layers 



