322 



THE CAT. 



[CHAP. x. 



The notochord appears to be formed from the mesoblast, but it 

 may be really of hypoblastic origin. While these changes are in 

 progress, a depression appears close to both 

 the cranial and caudal ends of the embryo 

 the head and tail folds and these extending 

 along each side of the embryo, the latter 

 comes to look somewhat like a small boat, 

 keel upwards, upon the surface of the ovum, 

 surrounded by the groove or depression 

 thus formed. The parts of this surrounding 

 groove between the head and tail folds are 

 called the lateral folds. At the outer margin 

 of this depression the surface begins to grow 

 up all round the embryo as a prominent 

 circumvallation. Meanwhile the mesoblast 

 extends outwards and downwards on each 

 side of the embryo, so forming what are 

 called the ventral plates and lamina vcntraks 

 (Fig. 147, A, Iv). As each ventral plate 

 descends it splits into two secondary lamina) 

 (one external, the other internal) called re- 

 spectively the SOMATOPLEURE and SPLANCHNO- 

 PLEURE (Fig. 147, B, so and sp). The distal 

 part of the somatopleure, where it extends 

 beyond the embryo into the circumvallation 

 before mentioned, ascends with that circum- 

 vallation around the groove about the embryo 

 to form the amnion, as explained below. The 

 splanchnopleure adheres closely to the hypo- 

 blast adjoining the yelk. That part of the 

 somatopleure which does not grow upwards, 

 comes ultimately to form the whole of the walls 



of the body and the limbs (Fig. 147, D). The splanchnopleure forms 

 the alimentary canal and structures adjacent to it. The cavity between 

 the somatopleure and splanchnopleure is the precursor of the future 

 peritoneal cavity (Fig. 147, D,^), and the innermost layer of the former 

 and the outermost layer of the latter form respectively the parietal and 

 the visceral layers of the peritoneum. Thus we have the body cavity 

 formed by a spontaneous longitudinal splitting of the mesoblast on each 

 side. The further extension of the somatopleure gives rise to the 

 AMNION. It is formed thus : the somatopleure of the mesoblast, carry- 

 ing the epiblast with it, rises up on all sides of the embryo at the 

 outer margins of the groove round the embryo (around the head, tail, 

 and lateral folds in the circumvallation before mentioned) and as each 

 ascending part is double, an extension of the primitive peritoneal cavity 

 becomes thus continued upwards on all sides of the embryo. A chamber 

 is at the same time thus gradually formed between the outer surface 

 of the embryo and the innermost (more and more bent over) surface 

 of this ascending, circumferential double layer, which thus gradually 



Fig. 146. EMBRYO MORE AD- 

 VANCED, SEEN SIMILARLY 

 ABOVE WITH PART OF THE 



BLASTODERM ATTACHED. 



The medullary groove is not 

 yet closed over, but at 

 the head end it is par- 

 tially divided into the 

 three central vesicles ; 

 the hinder end shows a 

 smaller enlargement. Six 

 primordial vertebra*, (pv) 

 are to be seen on each 

 side.- 



so. Epiblast, or upper laj'er. 



up. Lower layer of the blas- 

 toderm. 



