114 DE]'ELOPiMENT OF AMl'lIIOXUS. 



cavity therefore no longer opens by the blastopore to the 

 exterior, but it communicates with the medullary tube. 

 The blastopore has, in fact, become converted into the 

 iii'iiirntfric canal, joining the canal of the central nervous 

 system with the cavity of the alimentary system. This 

 remarkable contlition of things was first discovered b)' 

 KuwALEVSKY, who also found it in the Ascidians and in 

 a number of the higher Vertebrates. It has since been 

 found to occur in all classes of Vertebrates, including 

 man. 



Hitherto the body-wall of the embryo has consisted of 

 only two primary germ-layers, cctodcnii and oidodcrm. 

 At the stage now under consideration, a third interme- 

 diate layer, the vwsodt-n/i, has begun to put in its appear- 

 ance. The mesoderm arises in the first instance as a 

 series of paired lateral pouches of the archenteron. In 

 Fig. 62 the first two or three archenteric pouches are 

 distinctly visible. Before proceeding, however, to a more 

 detailed account of the origin of the nervous system and 

 of the mesoderm, we will trace briefly the changes in 

 external appearance which the embryos undergo up to the 

 time of the formation of the mouth. 



As the embryos are \-cry transparent, the external 

 appearance necessarily involves a good deal of the inter- 

 nal structure. 



The period of CDihryoiiic development may be defined as 

 commencing with the first cleavage of the ovum, and end- 

 ing with the perforation of the mouth, thus comprising 

 approximately the first thirty-six hours. During this 

 period the embryo does not take up independent nourisb.- 

 ment, but lives on the original food-yolk which was con- 

 tained in the egg. 



During the first few hours of its pelagic or free-swim- 



