THE LATEK EMBRYONIC DEVELOPMENT. 



69 



the muscles in an animal in which the skeleton is represented 

 merely by an elastic notochord. 



The development of new somites during the later stages of 

 embryonic life occurs very 

 slowly ; and at the time 

 of the formation of the 

 mouth, marking the close 

 of the period, there are not 

 more than fourteen or fif- 

 teen pairs. The elongation 

 of the body, which is so 

 marked a feature of the 

 later embryonic stages, is 

 due, not so much to addi- 

 tion of new segments, as to 

 lengthening of those al- 

 ready present ; and this 

 lengthening, as shown in 

 Figs. 33 and 34, principally 

 concerns the anterior or 

 oldest somites. 



A. The Alimentary Canal. 



After separation of the 

 somites and the notochord, 

 the archenteron, or, as it is 

 usually termed from this 

 time, the mesenteron, forms 

 a straight tube (Figs. 30 

 and 33, T), dilated at its 

 anterior end, but narrow 

 and cylindrical along the 

 greater part of its length. 

 It is closed in front, but 

 at its hinder end it com- 

 municates through the 

 neurenteric canal with the 

 neural tube, and so, indirectly, through the neuropore, with the 

 exterior. It is ciliated along its entire length, but no food particles 

 have as yet been observed in it prior to the formation of the mouth. 



