t ECHINOCYAMUS PUSILLUS. 27 



ment of Hensen was supported by Metschnikoff *) in his studies on 

 the development of the Echinoderms and Nemertines. He says in the 

 résumé, that in many Echinoderms after the commencement of the 

 invagination, in others, for instance Echinus lividus, before the invagi- 

 nation takes place, cells either lose their connection with the other 

 cells in such great numbers that they fill up the blastocœl, or they 

 occur in such small numbers that they are able to move by means of 

 the pseudopodia. Besides, it is plain from his researches that the wan- 

 dering cells arise either from the invaginated entoderm or from the 

 point where the invagination takes place. 



Some years afterwards Selenka ^) published his beautiful resear- 

 ches on the development of Holothuria tubulosa and Cucumaria planci, 

 where he points out that the mesenchyme originates from cells which 

 separate from the entoderm, either during the process of invagination 

 from the archenteron as in the former, or before this process from the 

 vegetative pole of the Blastula as in the latter. 



In 1879 the same author ') published his studies on the develop- 

 ment of the germinal layers and of the organic system of the Echi- 

 noids and here he was able to state the same to be the case in no 

 less than five different species. 



In 1883 Selenka *) again gives an account of the germinal layers 

 in some types of Echinoderms and shows that the mesenchyme origi- 

 nates in the Echinoids and Ophiurids from the entoderm, and in Synapta 

 from the same germinal layer, but not till a later stage and from the 

 end of the almost fully grown archenteron. 



Two years afterwards Metschnikoff ') in his new studies on the 

 Echinoderms pronounced more distinctly as to the views mentioned 

 above. He says that the wandering cells in the Echinoderms, as far 

 as is known from the Asteroids and Echinoids, arise from the ento- 

 derm or from that pole of the Blastula where the entoderm becomes 

 invaginated. 



1) Studien über die Entwickelung der Echinodermen und Nemertinen. — Mém. 

 de I'Acad. imp. d. sc. de S:t Pétersbourg. Sér. VII. T. XIV. Nio 8. 1869. p. 63. 



2) Zur Entwickelung der Holothurien (Holothuria tubulosa and Cucumaria 

 doliolum). — Zeitschr. f. wiss. Zool. Bd. 27. Leipzig 1876. p. 160—168. 



3) Keimblätter und Organanlage der Echiniden. 1879. p. 45 — 48. 



4) Die Keimblätter der Echinodermen. 1883. p. 44 — 46. 



5) Vergleichend -embryologische Studien. — Zeitschr. f. wiss. Zool. Bd. 42. 

 Leipzig 1885. p. 670. 



