MOEPHOLOGY OF CYCLORTOMATA. 06/) 



the spindles, tlio long axes of which are directed always parallel 

 to the surface of the layer. / have nowhere observed any traee of 

 either the proliferation or of the casting off of eells from the einhlaü 

 to give rise to the segmental duet. 



In the cloacal region, the formation of the segmental duct 

 o-oes on a little earlier than in the reojion next anterior to it. 

 In sj^ite of much effort, I fiiiled to observe the very beginning 

 of the formation at the cloacal opening, and I have nothing to 

 tell of its earliest stage. In the series of sections from which 

 figs. 77-89, are drawn, I can not yet find in the adjacent part 

 of the cloaca any trace of the duct ; but in the section repre- 

 sented in fig. 00 wdiich passes through the dorsal lip of the 

 blastopore of an embryo with about the same number of the 

 mesoblastic somites (34 or 35) as the one just referred to, the 

 duct already breaks through into the cloacal cavity {eo.sd.Y\ Fig. 

 91 represents the next ventral section which passes through the 

 dorsal part of the blastopore {hp.). As seen in these two sections, 

 immediately inside of the blatlopore [hp.], where the hypoblast 

 passes over into the epiblast, the walls of the cloacal cavity send out, 

 right and left, a symmetrical pair of diverticula-^ {c.dv.), forming 

 an acute angle, the inner side of which is a part of the enteric wall, 

 Avhile its outer side is the direct continuation of the epiblast. 

 The walls of this diverticulum pass over into the segmental duct 

 {sd.). The eoiiimunieation of the segmental duet with the eloaral 

 cavity is found, therefore, at the point where the epiblastie layer 

 of the lip is reflected inside and passes over into the hypoblast. 

 This point of communication is, however, shifted far inside and 



l)This opening is found in the same vertical plane as the n4th or 35th somite. 

 2)Tlie right diverticulum only is seen in figs. 90 and 91, the left one being ol)served in 

 another section which is nniigurcd. 



