THE EARLY HISTORY OF THE MESODERM. 81 



actual expanding edge of the mesoderm is quite irregular. The regularity shown in 

 figure 43 is entirely diagrammatic. 



The extent of the growth of the mesoderm over the extra-embryonic region of 

 the mammalian blastodermic vesicle is very variable. Usually it extends completely 

 around the vesicle, but in some cases, as in the rabbit, only part way (compare 

 page 52). 



The Origin of the Ccelom. The next step in the differentiation of the middle 

 germ-layer is the appearance of two slit-like cavities in it, one on each side. 

 These cavities do not extend across the median line, for when they appear there is 

 no mesoderm in the median line of the embryo. The ccelom is the anlage of the 

 body-cavity, and in part persists in the adult as the pericardial, pleural, and ab- 

 dominal cavities. Certain parts of its walls share in the production of muscles 

 and of the excretory organs. The complete history of the ccelom 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 ccelom have been exactly fol- 

 lowed 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 ccelom. 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 peri- 

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

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

 the head of the embryo and are known as the amnio-cardiac vesicles (Fig. 131, 

 A.c.v). (Compare also the account of the splanchnocele, page 87.) 



There are very great variations in the development of the ccelom in mammals. 

 In some cases the coelom grows so as to appear at an early stage in the body of 

 the embryo (Fig. 37). 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 ccelomate space between the 

 two mesodermic layers. This space we call the extra-embryonic ccelom. These 

 relations are illustrated in figure 45. 



As soon as the ccelom 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-vessels or 

 blood in the somatic mesoderm. The inner layer is called the splanchnic mesoderm. 



