MORPHOLOGY OF CYCLOSTOMATA. 327 



just in the process of being- constricted ofif from the myotome, and we 

 can not decide by this case alone which segment (whether the an- 

 terior or the posterior) is the further developed ; a comparative 

 study of other examples shows that the separation in the posterior 

 segment follows that in the anterior. The state of the mesoblast in 

 the next posterior segment, i.e., the tenth segment (fig. 31), is quite 

 different from that just described ; it is in a more primitive condition 

 of development. The Anlage of the pronephros {a.scl.) presents only 

 an indication of constriction, — a feature which we have observed 

 repeatedly in embryos of younger stages (compare with figs. 2, 5, 6, 

 14, and 16). From this segment backwards, a few segments show 

 almost the same condition. Still further posteriorly, the structure 

 of the mesoblast can not be readily observed, since the planes of 

 sections incline by degrees in the cranio-caudal direction, owing, as 

 above stated, to the bending of the longitudinal axis of the embryo. 



In all the segments mentioned above, the lateral plate {Im.) 

 consists of a loose tissue of cells of variable shape, and the Anlage 

 passes over suddenly into the lateral plate just as in the embryos 

 described in the foregoing pages. 



Ill this stage, therefore, the Anlage of the pronei^hros is com- 

 pletely separated from the myotome in 4 somites, i.e., from, the 

 fourth to the seventh inclusive ; and these are connected icith one 

 another by the intersomitic solid cord. In the following 4 or 5 

 somites, the constriction is just going on, while in a few of still 

 more posterior somites it is indicated merely by a slight dejyressioii 

 in the parietal layer of the mesoblaH. 



Period 2. 



Tn the embryos which belong to Stage in, we observe a 

 decided advance in several respects. Figs. 32-50 represent a series 



