ECHINOCYAMÜS PUSILLUS. 29 



that in the Echinoderms the mesenchyme mostly has its origin from the 

 entoderm, but that the ectoderm also can give rise to it. Lastly in the 

 year 1891 the same investigator') insists that the mesenchyme in Cucu- 

 maria planci (= doliolum) is due to the ectoderm as well as the ento- 

 derm, an opinion opposite to that expressed by Selenka. 



Finally, we must not forget a third view first pronounced b}^ 

 Metschnikoff in his first mentioned work, and confirmed recently hj 

 Russe ^) who found in Amphiura squamata that, as soon as the two 

 layers of the gastrula have been produced by a process of delamination, 

 the mesenchyme begins to originate by means of a new act of delami- 

 nation taking place in the ectoderm '). If this statement of Russe is 

 right in all respects, we have to do with a very remarkable exception 

 from the general rule. 



If we consider the different views of the origin of the mesenchyme 

 as expressed by the above authors, we find that the mesenchyme origi- 

 nates in three ways in the Echinoderms: viz. by immigration from the 

 entoderm alone in most Echinoderms, or by immigration from the ecto- 

 derm as well as from the entoderm as is the case in a few forms, or 

 finally by a process of delamination from the ectoderm known only in 

 Amphiura squamata (and Psolinus brevis?). 



Before I go on to relate my own investigations with regard to 

 Echinocyamus, I must take into consideration the opinion promulgated 

 by Selenka, Hatschek and Fleischman, that the mesenchyme should 

 originate from only two or four cells of the entoderm named »arche- 

 mesenchyme cells», a view opposite to that expressed by Metschnikoff, 

 KoRSCHELT and Ludwig. 



The »arche-mesenchyme theory» is mainly due to Selenka ^). 

 Hatcshek ^), it is true, was the first to call attention to the existence 



1) Zur Entwickelungsgeschichte der Holothurien. — Sitzungsb. d. K. preuss. 

 Acad. d. Wiss. zu Berlin. XXXII. Berlin 189L p. 605. 



2) Die Keimblätterbildung bei Amphiura squamata (Sars). — Zool. Anzeiger. 

 N. 377. 1891. p. 407. 



3) KowALEVSKY also in his »Beiträge zur Entwickelungsgeschichte der Ho- 

 lothurien» (Mém. d. I'Acad. impér. d. sciences de S:t Petersbourg. Sér. VIL T. XI. 

 N:o 6. 1867. p. 3) informs us that in Psolinus brevis Foebes the mesenchyme origi- 

 nates by a kind of delamination, but his account of this process is not so clear as 

 to exclude all doubts of the correctness of his observations. 



4) Die Keimblätter der Echinodermen. 1883. p. 43—48. 



5) Ueber Entwickhingsgeschichte von Teredo. — Arbeiten aus dem Zool. 

 Instit. Wien. III. 1881. p. 30. 



