REPRODUCTION. 



697 



dermal cells in the region of the chordal invagination spread between ecto- 

 derm and entoderm and form the anlage of the mesoderm. These cells by 

 rapid proliferation soon separate ectoderm and entoderm and join the 

 extra-embryonic mesoderm. The separation of ectoderm and entoderm is 

 complete except in the regions of the bucco-pharyngeal and cloacal membranes. 



Upon each side of the neural groove the mesoderm becomes transversely 

 grooved on its ectodermal surface, forming a number of successive block- 

 like masses called primitive somites or segments. Of these there are thirty- 

 eight of the trunk and possibly four for the head region. Each segment 

 consists of three parts the sclerotome, the myotome and the dermatome. 

 Lateral to the somite is a thickened mass of mesoderm, the intermediate cell- 

 mass, that laterally splits into two layers, the outer accompanies the ecto- 

 derm forming the somatopleure which gives rise to the body wall, the inner 

 joins the entoderm forming the splanchnopleure from which the gut-tract, 

 vitelline duct and yolk-sac are derived. 



Fetal Membranes. As the primitive streak and neural groove are form- 

 ing, the extra-embryonic mesoderm that lies beneath the trophoderm invades 

 the trophodermal villi forming thus 

 the chorion with its villi. Gradually 

 the mesoderm of the roof of the 

 amniotic cavity splits into two layers, 

 the upper constituting chorionic meso- 

 derm while the under one attached to 

 the ectoderm of the amniotic forms 

 with the latter the amnion. In the 

 chick and some mammals, the amnion 

 is derived from the somatopleure in the 

 folding off of the body. In amniotes 

 the amniotic cavity is at first small 

 but rapidly increases in size. It con- 

 tains a clear, transparent liquid, the 

 amniotic fluid, which amounts to about 

 one liter at term; it serves to protect 

 the fetus during gestation and at parturition it dilates the os cervicis, and 

 flushes the birth canal. This liquid is derived mainly from the blood as it 

 contains albumin, sugar, fat and inorganic salts. Traces of urea indicate 

 that some of its constituents are derived from the embryo itself. 



The caudal end of the embryonic area is left connected with the chorion 

 by a heavy band of mesoderm termed the belly-stalk, to which the caudal 

 part of the amnion is attached. The entoderm is invaginated into the 

 belly-stalk for a short distance constituting the allantois of higher forms. 

 In oviparous forms the allantois grows out between the closing somatopleuric 

 folds that form the body-wall and constitutes a free sac upon which vessels 

 (allantoic arteries and veins) develop from the embryo. This sac then 

 spreads beneath the white shell membrane forming the organ of nutrition 

 and respiration of these forms during the last half of their incubation periods. 

 In mammals the extra-embryonic portion of the allantois is of little 

 importance. (Fig. 351.) 



FIG. 351 . HUMAN EMBRYO AND ITS EN- 

 VELOPES AT THE END OF THE THIRD MONTH, 

 (Dalton.} 



