STUDIES IN MAMMALIAN EMBRYOLOGY. 525 



considerable cenogenetic changes, we find mesoblast to arise 

 from and be connected with the selfsame strips of tissue from 

 which we have seen the notochord to originate. 



Both the protochordal plate and the protochordal wedge 

 give rise to lateral wings of mesoblast, which posteriorly pass 

 into the mesoblast that is segregated from the coalesced lips 

 of the blastopore along the gastrula ridge. The duplicity in 

 the origin of the notochord, here ascribed to precocious segre- 

 gation of a portion of the hypoblast, very naturally also 

 applies to the mesoblast that originates in the corresponding 

 regions. At the same time it appears equally natural that 

 when the fusion of the two protonotochordal halves has come 

 about, the two pairs of mesoblast wings should also reunite. 



Direct comparison between Amphioxus and Mammalia is 

 less easy when we come to consider the mesoblast that arises 

 from the gastrula ridge and that which takes its origin from 

 the peripheral hypoblast. When the embryo folds off from 

 the yolk-sac this latter hypoblast constitutes the greater part of 

 the wall of the gut (Darmentoblast). As far as we can judge 

 from Hatschek's and Kowalevsky^s observations, nearly all the 

 mesoblast of Amphioxus is derived from the archenteric 

 diverticula above alluded to. Hatschek further mentions and 

 figures two larger polar cells of mesoblast at the hind end of the 

 embryo, right and left of the blastopore, but we only learn 

 in very general terms that they participate in the formation 

 of mesoblast for the caudal region (1. c, p. 35),^ 



Here, however, the Cyclostomata and the Amphibia furnish 

 us with data that are valuable for the explanation of the phe- 

 nomena in the Mammalia. 



To Calberla, Scott (25a), Nuel (22), and Shipley (28) we 

 are indebted for the facts as they present themselves in Cyclo- 

 stomata; to Scott and Osborn (25), O. Hertwig (12), O. Schultze 

 (26), and many others to those which are noticed in Amphibia. 

 Still there is divergence of opinion amongst these authors on 



^ " Wir werden selien dass diese Zellen die stets den hinteren Korperpol 

 bezeicbnen bei der Bildung des Mesoderms den liiuteren Abschluss desselben 

 bildeu." 



