30 Hjalmar Théel, 



of two closely adjacent cells at the vegetative pole of the Blastula in 

 Toxopneustes lividus, which according to him should multiply by division, 

 enter into the segmentation cavity and originate the whole mesenchyme. 

 However, some years afterwards Selenka^,_ again finds the two primordial 

 cells, which were distinguished by their shortness from the surrounding 

 higher cells, in many Echinoderms viz. Strongylocentrotus lividus, Sphser- 

 echinus granularis, Echinus microtuberculatus, Ophioglypha lacertosa and 

 even in Synapta digitata, although in the latter they were placed at the 

 end of the fully grown archenteron. On account of these observations 

 be believed himself justified in founding the theory, that in the Echino- 

 derms, two primordial or arche-mesenchyme cells are present which 

 either are placed bilaterally at the vegetative pole of the larva (Echi- 

 nids and Ophiurids) or at the end of the archenteron (Synapta), and 

 that in the Echinoids (Asteroids? and Ophiurids?) they divide and give 

 rise to two »mesenchyme bands» homologous to the mesoblastic bands 

 known in the Mollusks, Worms etc., which in company with the two pri- 

 mordial cells enter the blastocoel. Fleischman '), who has studied the 

 first developmental processes in Echinocardium cordatum, differs slightly 

 from Selenka, insisting that there are four arche-mesenchyme cells in 

 that species. 



This theory of Selenka has been discussed by the three above 

 mentioned investigators, who deny the existence of the arche-mesen- 

 •chjj-me cells as well as of the mesenchyme bands. They explain that 

 the detruncated cells at the vegetative pole are nothing but common 

 blastomeres in a state of division and multiplication, when as a rule 

 they become shorter. 



After this survey of the different opinions which exist with regard 

 to the origin of the mesenchyme, I may be allowed to show the re- 

 sults which I have gained during my own researches into the develop- 

 ment of Echinoc3^amus. In these studies 1 found it useful to examine 

 the larvse in a living state as well as after they had been fixed and 

 stained. The most suitable fixing fluids were Flemming's fluid, or a 

 rather dilute solution of osmic acid. Besides the whole larvae, I have of 

 course examined many series of sections of them in their different stages 

 of development. 



As soon as the blastosphere has changed into a somewhat elon- 

 gate Blastula, PL 11^ fig. 36 and PL III, fig. 50—51, the cells at the 



l^Die Entwicklung des Eies von Echinocardium cordatum. 1888. p. 139. 



