114 



THE EARLY DEVELOPMENT OF MAMMALS. 



end of the uterine epithelial cell, so that the two layers are closely fitted together, 

 cell for cell. 



The Bladder. The allantois extends from the cloaca to the umbilicus, and 

 beyond the umbilicus into the umbilical cord. It comprises, therefore, an embry- 

 onic and an extra-embryonic portion. The former is the anlage of the urogenital 

 sinus, the urethra, and the bladder. The embryonic portion is always united to 

 the abdominal wall (Fig. 210), the mesenchyma, which surrounds the entodermal 

 allantois, All, and the umbilical arteries, S.um, being fused in the mid-ventral line 



Ut.Ep. 



Conn. 



EC. 



Mes. 



FIG. 65. PIG, 15.0 MM., SERIES 135, SECTION 58, TO SHOW THE RELATIONS OF THE CHORION TO THE UTERUS. 

 Conn, Connective tissue of the uterus. EC, Chorionic ectoderm. Mes, Choriomc mesoderm. Ut.Ep, Uterine 

 epithelium. Ve, Chorionic blood-vessel. X 350 diams. 



with the mesenchyma of the somatopleure. This connection is retained throughout 

 life. The opening of the Wolffian ducts into the allantois is established very 

 early. The ureters at first open into the Wolffian ducts, but they soon migrate 

 so as to open separately directly into the allantois (bladder) above the ducts. 



The Trophoderm. 



Trophoderm is the name applied to the special layer of cells developed on the 

 outer surface of the ectoderm of the mammalian blastodermic vesicle. It has as 

 yet been observed only in unguiculates. The trophoderm layer may be devel- 

 oped over the entire surface of the ovum, as in man, or over only a portion 

 thereof, as in the rabbit and cat. Its principal known function is to destroy the 

 tissues of the uterus of the mother with which it comes in contact. The destruc- 

 tion of the tissue is supposed to serve two purposes: First, to supply nutrition to 

 the embryo. It is from this supposed function that the layer derives its name of 



