ON THE DIPLOCHORDA. 397 



Comparison with the Work of otliers. 



In comparing these facts with the previous work of others, 

 one can divide them into two series. The first of these 

 consists of facts which have been observed by external exami- 

 nation and connected with the external differentiation of the 

 embryos, including the phenomena of segmentation, gastru- 

 lation, and the fate of the blastopore ; the second are the 

 changes undergone by the tissues within the embryo, 

 especially connected with the phenomena of gastrulation and 

 the formation and subsequent fate of the mesoblast. With 

 regard to the first series all observers agree very closely. 

 Segmentation has in all cases been recognised as being nearly 

 equal, and gastrulation embolic. The blastopore appears to 

 survive (in part at least) as the mouth in all the species, but 

 all do not recognise the fusion of the blastoporic lips pos- 

 teriorly. Metschnikoff (14) followed and figured the fate of 

 the blastopore in detail, and he notes that it is at first round, 

 then becomes reduced in size, and then presents a pointed 

 oval outline. He also describes a median longitudinal furrow 

 running backwards from the reduced blastopore to the 

 posterior end of the embryo. Caldwell (3) followed with 

 another account, which describes a similar ventral furrow (his 

 so-called primitive streak), but in addition he finds a posterior 

 pit, which he regards as the posterior part of the blastopore. 

 Metschnikoffs account would be quite as true for Phoronis 

 Buskii as for his own species. Schultze (16), working upon 

 a species in the Black Sea, finds no trace of this ventral 

 furrow. 



The difference may all be probably accounted for as due 

 either to defective observation or to specific variability. 



Coming to the second series of phenomena a very different 

 condition prevails. All previous observers with the excep- 

 tion of Caldwell appear to have relied upon the observations 

 of entire embryos in optical section, and this method, when 

 applied for the elucidation of important internal changes, can 



