THE DEVELOPMENT OF THE NYMPH. 335 



mesoblast of the disc, and extend into the parablast. I have 

 as yet been unable to determine the fate of these cells, and am 

 doubtful whether they are of parablastic or mesoblastic origin. 

 I am tempted to believe, from their arrangement in strings, that 

 they are developed from the mesoblast, and are rudimentary 

 muscle cells ; but as I am uncertain as to their nature I shall 

 term them intermediate cells, from their position between the 

 mesoblast of the disc and the parablastic plug which fills the 

 provisional cavity. 



The stellate parablast extends over the outer surface of the 

 provisional capsule, and when the latter is distended by the de- 

 velopment of the disc, ultimately entirely replaces its epithelial 

 elements. 



The various views which have been held as to the origin of 

 the mesoblast of the discs are sufficient evidence of the diffi- 

 culties which beset the investigation. I believe, however, that 

 the recognition of the true character of the parablast, and its 

 extreme importance in the developmental process, will do much 

 to clear up many difficulties, and affords the clue to a recon- 

 ciliation of many conflicting views. 



Viallanes everywhere figures and describes my parablast as 

 the mesoderm of the discs, and naturally arrives at the follow- 

 ing conclusion : ' M. Ganin thinks that the mesoderm in each 

 disc is derived from the exoderm, but a certain number of new 

 facts lead me to think this is not general, but that in many 

 cases the mesodcnn of the discs is jormed from embryonic cells (blood 

 corpuscles, etc.] scattered in the general body cavity.' 



Kowalevski, on the other hand, describes and figures my 

 intermediate cells as mesoderm, and thinks they are developed 

 from wander-cells. Van Rees entirely discards the term ' meso- 

 derm ' and uses the word ' mesenchyme.' He says : ' As matters 

 stand, I must declare myself in favour of Ganin's hypothesis, 

 and hold that this tissue arises from the cells of the epithelium 

 of the discs. It is on this account that I use the term " mesen- 

 chyme " in the place of the old term " mesoderm," which is not 

 only less definite, but incorrect, if we accept the views enunciated 

 by the brothers Hertwig concerning its origin in their ccelom 



