110 



DE VEL OPMENT. 



of vascular connection between the embryo and the uterine tissues, is formed. In 

 fact, the umbilical vesicle provides for the nutrition of the foetus from the ovum 



FIG. 85. Magnified view of the human embryo of four weeks, with the membranes opened. (From Leish- 

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

 attachment to the intestine, c. The yilli of the chorion. m. The amnion opened, u. Cul-de-sac of the allan- 

 tois, 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. I. The liver, o. The upper; , the lower limb. w. Wolfflan body, in front of 

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



itself, while the allantois is the channel whereby it is nourished from the uterine 

 tissues. The umbilical vesicle, containing fluid, remains visible, however, up to 

 the fourth or fifth month, with its pedicle and the omphalo-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 yolk into a 

 number of nucleated cells or " spherules." (2) The accumulation of fluid within 

 the ovum, and the arrangement of the spherules around the fluid on the 

 internal surface of the vitelline membrane, forming a second membrane, the 

 " blastodermic membrane." (3) The separation of the blastodermic membrane into 

 three layers, named, from within outAvard, the "hypoblast," the "mesoblast," and 

 the "epiblast." (4) The formation of an elongated, oval-shaped disk, called the 

 " area germinativa." (5) The appearance in the centre of the area germinativa of 

 a delicate line or furrow, running longitudinally, and called the ' primitive trace." 

 (6) The formation of a distinct groove in the situation of this primitive trace, 

 caused by the growing-up of the cells on either side of it, so as to form two longitu- 

 dinal ridges, called the "laminae dorsales." (7) The increase and incurvation of 

 these laminae dorsales, until they meet behind, enclosing a canal lined by epiblast. 

 The canal is the neural canal, and from the epiblast which lines it the nervous 

 centres are developed. (8) The formation, in the hypoblast immediately under this 

 canal, of a continuous rod-shaped body, the "chorda dorsalis," or "notochord." 

 (9) The formation from the mesoblast, on either side of the notochord, of a longi- 



