FIFTH PERIOD OF DEVELOPMENT. 163 



The interruption of the communication between 

 the archenteron and the neural canal takes place, 

 more or less simultaneously with, or perhaps some- 

 what later than, the perforation of the anus. 



The anus perforates outward ventrally from this 

 opening of communication, which represents the last 

 that is left of the gastrula-mouth, it is at the same 

 time moved unsymmetrically to the left side of the 

 body. The perforation is immediately in front of 

 the caudal fin, which bounds the hinder end of the 

 body. 



Kowalevsky has most admirably described these 

 relative positions of the anus in his " Further 

 Studies." 



FURTHER ALTERATIONS. 



The form of the body becomes more marked. Its 

 elongation continues taking place, through extension 

 of the already formed somites, for the multiplication 

 of the latter is now, as is the case with all internal 

 processes of development, extraordinarily delayed. 

 Indeed, during this proportionately lengthy period, 

 only one additional mesoblastic somite is formed, this 

 being the fifteenth. The lateral compression also 



