76 THE EARL Y DE VEL OPMENT OF MAMMALS. 



abdominal cavities. Certain parts of its walls share in the production of mus- 

 cles and of the excretory organs. The complete history of the coelom is very 

 complex. As the coelomatic cavities appear, the cells bounding them take on a 

 distinctly epithelial character. This limiting layer is termed the mesothelium. 



The earliest phases in the development of the coelom have been exactly 

 followed only in a very few instances. In these it has been found that numerous 

 fissures appear in the mesoderm and unite themselves so as to form a network of 

 channels which grow, and produce by their fusion the coelom. The fusion occurs 

 so that two cavities are developed, one on either side, and parallel with the axis 

 of the embryo. As the head of the embryo grows the two cavities grow into its 

 cervical end, following the penetration of the mesoderm, and unite so as to form 

 below the developing pharynx a single median cavity, the anlage of the future 

 pericardial cavity. In the Sauropsida and in many mammals the pericardial 

 coelom merges into two large expansions of the body-cavity which lie just along- 

 side of the head of the embryo and are known as the amnio-cardiac vesicles (Fig. 

 167). (Compare also the account of the splanchnocoele, page 82.) 



There are very great variations in the development of the coelom in mam- 

 mals. In some cases the coelom grows so as to appear at an early stage in the 

 body of the embryo (Fig. 27). In other cases it is developed in the entire extra- 

 embryonic region of the blastodermic vesicle before it is developed in the embryo 

 proper. This condition has been observed in primates, including man. It 

 results in the formation of a layer of mesoderm surrounding the yolk-sac, and 

 another layer underlying the extra-embryonic ectoderm, with a wide coelomatic 

 space between the two mesodermic layers. This space we call the extra-embry- 

 onic coelom. These relations are illustrated in Fig. 31. 



As soon as the coelom has appeared the mesoderm is divided into two layers, 

 an outer and an inner. The outer layer is in close contact with the ectoderm. 

 It is called the somatic mesoderm. The inner layer is in close contact with the 

 entoderm; it includes the entire angioblast, there being in early stages no 

 blood-yessels or blood in the somatic mesoderm. The inner layer is called the 

 splanchnic mesoderm. 



Somatopleure and Splanchnopleure. 



The somatic mesoderm, together with the overlying ectoderm, constitute 

 the somatopleure or primitive body- wall. The splanchnic mesoderm, together 

 with the underlying entoderm, constitute the splanchnopleure. The somato- 

 pleure and splanchnopleure are, to a large degree, the elementary anatomical 

 parts out of which the adult structure is produced. Although they each com- 

 prise cells belonging to two germ-layers, they nevertheless develop each almost 



