DEVELOPMENT OF MESODERM 341 



pancreas), of the middle ear and Eustachian tube, of the 

 thymus and thyroid bodies, of the bladder and first part of 

 the male urethra and of the entire female urethra. 



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

 forms, such as bone, dentine, cartilage, lymph, blood, fibrous 

 and areolar tissue; (2) muscular tissue; (3) all endothelial 

 cells; (4) the spleen, kidney and ureter, testicle and its ex- 

 cretory ducts, uterus, Fallopian tube, ovary and vagina. 



The Embryonal Area. Soon after the germ reaches the 

 uterus (probably) there appears on its surface an oval whit- 

 ish 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 acces- 

 sory. Longitudinal division of this area is supposed to give 

 rise to twins of the same sex and of almost identical struc- 

 ture. 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 sur- 

 face 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 primi- 

 tive 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 em- 

 brace the germ, but is deficient opposite the embryonal area. 



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

 thickened 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 meso- 

 derm unite to form the splanchnopleure. The interval left 

 between the somatopleure and splanchnopleure is the 



