MOEPIIOLOGY OF CYCLOSTOMATA. 331 



the Anlagen fourth and fifth, and in fig. 49, one hetween the 

 Anlagen fifth and sixth. 



This inequality in the length of the intersomitic solid cord 

 is, I believe, due to differences in the degree to which the canali- 

 zation within the Anlage has extended into the connecting cord. 

 In the anterior section of the pronephros, this process has already 

 proceeded to some extent into the interior of this cord, while in 

 the posterior, the cavity is still confined entirely within the 

 Anlage itself. The whole system of the pronephros at the present 

 condition may be compared to a bamboo-cane with nodes and 

 internodes ; in the anterior section of the system, the nodal septum 

 has Ijecome very thin, while it has a considerable thickness 

 in the the posterior. As will be shown further on, all these 

 septa entirely disappear later when the collecting duct is fully 

 established. 



From the fact mentioned above, it Avill be easily seen that the 

 })rocess of canalization in the pronephric system of Peiroviyzoii 

 begins in the internal cavity of the pronephric Anlage in each 

 segment and is extended into the intersomitic connecting cord. 

 The direction in which this process proceeds seems, generally 

 speaking, to be from the anterior section to the posterior ; for 

 in most cases, not only the internal cavity in each Anlage is 

 spacious anteriorly and narrowed posteriorly, but the cavity in 

 anterior somites is extended more, or canalization goes on further, 

 than in the posterior section of the system ; although the pro- 

 gress in the opposite direction is occasionally met with. 



From the tenth somite backwards, five or six segments show 

 the same condition of the mesoblast as in the eighth and ninth 

 somites, after which the series can not be studied, owing to the 

 inclination of the planes of sections, referred to above. 



