526 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Sponges and Coelenterata ; it is found in some round 

 and flat worms, and in Echinoderms, but is very rare 

 in Insects, Molluscs, and Chordates, Amphioxus, in- 

 deed, being the only representative in the last- 

 mentioned phylum. 



Fig. 215. The Segmentation of Amphioxus. 



A, Stage with two, B, with four equal segment?. In c each of the four segments 

 has been divided horizontally ; D, a later stage in which a single layer of 

 equal cells encloses a central segmentation cavity ; B, the cavity (sy) is seen 

 in optical section. (After Kowalevsky.) 



The influence of food yolk on the segmentation has 

 been stated by Balfour in the following terms : " The 

 rapidity with which any part of an ovum segments 

 varies ceteris paribus with the relative amount of 

 protoplasm it contains ; and the size of the segments 

 formed varies inversely to the relative amount of 



