THE CCELOM. 321 



THE DEVELOPMENT OF THE BODY CAVITY AND 

 MUSCULAR SYSTEM. 



1. The Body Cavity or Ccelom. 



The body cavity or coelom is formed, as described on p. 243, 

 towards the end of the first day, by the splitting of the meso- 

 blast, or rather by rearrangement of the mesoblast cells into 

 two layers, upper or somatic and lower or splanchnic respec- 

 tively ; the body cavity being the narrow chink-like space 

 between the two layers. 



In the later stages, the body cavity enlarges considerably, by 

 further separation of its walls. It extends the whole length of 

 the body, but does not reach into the head, stopping at the 

 first mesoblastic somite or protovertebra. 



The two halves, right and left, of the body cavity are at first 

 separate ; but as the ventral body-wall of the embryo is com- 

 pleted, by constriction of the embryo from the yolk, the two 

 halves meet and open into each other across the mid-ventral 

 plane. Prom the mode of its formation, the body cavity is 

 directly continuous with the space between the two layers, 

 inner and outer, of the amnion (Figs. 11:2 and 129, an) ; and 

 it is owing to this continuity that the allantois is able to pass 

 out beyond the limits of the embryo, and spread over its back 

 (Figs. 100 and 101). 



The mesoblast cells lining the body cavity become the peri- 

 toneal epithelium ; from this epithelium the genital organs, 

 i.e. the ovary or testis, are developed ; and from it, or in close 

 relation with it, the tubules of the Wolffian body are formed. 



2. The Pericardial Cavity. 



There is at first no separate pericardial cavity ; the heart 

 lying in the anterior part of the general body cavity, ventral to 

 the oesophagus and pharynx (Fig. 112). About the end of 

 the second day a septum begins to form, which shuts off the 

 ventral portion of the body cavity, in which the heart lies, at 

 first partially and ultimately completely fi-om the general body 

 cavity ; and this shut-off portion becomes the pericardial cavity. 

 The septum is formed in the following manner. 



Opposite the hinder end of the heart the Cuvierian veins 



Y 



