340 REPRODUCTION 



From the mesoderm: (i) Connective tissue in all its forms, such 

 as bone, dentine, cartilage, lymph, blood, fibrous and areo- 

 lar tissue; (2) muscular tissue; (3) all endothelial cells; (4) 

 the spleen, kidney and ureter, testicle and its excretory ducts, uterus, 

 Fallopian tube, ovary and vagina. 



The Embryonal Area. Soon after the germ reaches the 

 uterus (probably) there appears on its surface on oval whitish 

 spot, the embryonal area. The impregnated ovum is still in the 

 shape of a vesicle. It is from the embryonal area alone that the 

 body is developed. The other parts are accessory. Longi- 

 tudinal division of this area is supposed to give rise to twins of 

 the same sex and of almost identical structure. Running in the 

 long diameter of the embryonal area is a marking, the primitive 

 streak, in which is a longitudinal depression, the primitive 

 groove. (Fig. 97.) These surface markings are caused by 

 thickening of the ectoderm. (Fig. 98.) 



Development of Mesoderm. It is about this time that the 

 mesoderm makes its appearance. It begins under the primitive 

 groove and extends in all directions. It originates from both 

 ectoderm and entoderm, and lies between them. In the median 

 line the three layers are closely united to each other. (Fig. 98.) 

 At first the mesoderm does not completely embrace the germ, 

 but is deficient opposite the embryonal area. 



Fig. 94 shows that the cells of the mesoderm make up a thick- 

 ened mass near the median line, but farther away they constitute 

 two distinct lamellae. The mass near the median line is the 

 vertebral or axial plate. The outer of the lateral lamellae is the 

 somatic mesoderm; the inner is the splanchnic mesoderm. The 

 ectoderm and somatic mesoderm unite to form the somatopleure; 

 the entoderm and splanchnic mesoderm unite to form the splan- 

 chnopleure. The interval left between the somatopleure and 

 splanchnopleure is the celom, or body cavity. (Fig. 98.) The 

 great serous cavities of the body are developed from it. 



Beginning Differentiation. It thus appears that the embryo 



