822 THE CHICK. 



cross the body cavity transversely, in order to reacli the sinus 

 venosus from the somatopleure ; and it is from the walls of the 

 Cuvierian veins that the pericardial septum arises, as a thin 

 sheet of connective tissue which grows forwards and upwards 

 to the under surface of the fore-gut, and obliquely downward- 

 and backwards to the ventral body wall, which it meets a little 

 way behind the heart. 



The lungs lie in two pocket-like diverticula of the body- 

 cavity which extend forwards along the sides of the oesophagus 

 (cf. Fig. 163). These pleural cavities at first lie dorsal to the 

 pericardial cavity, but, as they gradually enlarge to make room 

 for the lungs, they spread ventralwards over the pericardial 

 cavity, between it and the body walls. The pleural cavities 

 in the bird remain throughout life continuous at their hinder 

 ends with the general body cavity. 



3. The Muscular System. 



The mesoblastic somites or proto- vertebras (cf p. 244) are 

 paired, cubical blocks of mesoblast, arranged in series along the 

 sides of the spinal cord, and formed by transverse division of 

 the vertebral plates of the mesoblast (Figs. 110, 111, 113, ms). 



The somites are at first hollow ; their cavities are to be 

 regarded as parts of the ccelom ; and in the case of the anterior 

 three or four somites the cavities are for a time actually con- 

 tinuous with the general body cavity. The walls of the somites 

 are at first of nearly uniform thickness on all sides, but during 

 the second and following days they thicken very unequally, and 

 undergo further changes, by which they give rise to the greater 

 part of the voluntary muscles of the trunk, and to the elements 

 from which the vertebral column is developed. These changes 

 are effected as follows. 



During the second and third days (Fig. 129) the ventral 

 walls of the somites thicken very considerably, so that the 

 whole embryo is increased greatly in depth, from the dorsal to the 

 ventral surface, and the cavities of the somites, CM, become 

 situated, not in their centres, but close to their dorsal surfaces. 

 The dorsal part of each somite, inclosing the cavity, or myocoel, 

 CM, now separates off from the underlying mesoblast, as the 

 muscle-plate, mp : the walls of the muscle-plate consist of cells of 

 an epithelial character, closely packed side by side, and contrast 



