I 14 DEVELOPMENT OF AMPHIOXUS. 



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 

 neurenteric canal, joining the canal of the central nervous 

 system with the cavity of the alimentary system. This 

 remarkable condition of things was first discovered by 

 KOWALEVSKY, 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, ectoderm and cndodcrm. 

 At the stage now under consideration, a third interme- 

 diate layer, the mesoderm, 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 very transparent, the external 

 appearance necessarily involves a good deal of the inter- 

 nal structure. 



The period of embryonic 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 nourish- 

 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- 



