ON THE D1PI,OOHOHDA. 377 



to conclude that the mesoderm of the adult arises from 

 mesenchyme cells. In this opinion he is supported by 

 FcEttinger (7), who finds what he considers early mesen- 

 chyme cells (" le premier element mesodermique") in as 

 early a stage as that of eight blastomeres. He followed 

 the segmentation processes from the earliest condition to 

 gastrulation. Like Metschnikoff, he relied upon optical 

 sections of the entire embryos, and also subjected them to a 

 course of treatment with acetic acid. 



In 1885 appeared a short paper on the early stages of 

 Phoronis by Caldwell (3). This observer was the first to 

 apply the method of sections to the elucidation of the subject 

 in hand. His paper is almost entirely concerned with the 

 origin of the mesoblast. Briefly, he dismisses Kowalevski's 

 statements by supposing that he was deceived by the ecto- 

 derm cells being darker at the base, and those of Metschni- 

 koff and Foettinger by explaining their " mesenchymatous" 

 cells as being "amoeboid processes of the eudoderm cells 

 growing into the segmentation cavity," or possibly other 

 non-nucleated bodies which are to be found in the blastocoele. 

 He concludes, '• I have observed them frequently, but it is 

 certain that they have nothing to do with the true meso- 

 blast." 



His papers and figures have been introduced into numerous 

 text-books, and are well known, so that we need merely 

 repeat here his summary (p. 25). 



1. The blastopore gives rise to both mouth and anus. 



2. The mesoderm arises in an anterior pair of entoblastic 

 modified diverticula, and in a posterior pair of ectoblastic 

 diverticula connected by a few mesodermic cells derived from 

 the middle of a primitive streak. 



3. The nephridial openings to the exterior are parts of the 

 blastopore. 



In 1890 Eoule (16) attacked the subject, and formed no 

 exception to the others in differing remarkably from his 

 predecessors. His material was derived from Phoronis 

 sabatieri. 



