NOTES ON EMBRYOLOGY AND CLASSIFICATION. 



419 



parentera, from masses of cells separating from the primitive 

 enterou. In the Vertebrates it is most clear that only a 

 small axial tract, if any, of the cells which give rise to 

 muscular-skeletal tissues originates from ectoderm. The 

 original mode of formation by delamination has been lost. 

 The parenteric growths, separating early whilst all the cells 

 of the embryo are neutral in appearance, form a large in- 

 termediate sheet of cells — the much debated '' mesoblast " or 

 " mesoderm " — and from this layer, muscular and skeletal 

 cells, vascular and coelomic epithelium, all alike develop. 

 Compare with this extreme state of modification the more 

 ancestral mode of development of the corresponding parts of 

 the tissues of Holothurians, as described in the valuable and 

 important memoir of Selenka (' Zeitsch. Wiss. Zool./ vol. 

 xxvi). In Holothuria, the two elements which in the 

 Vertebrates are confused to the eye, and which are too 

 often mentally confused under the one name of meso- 

 blast, are seen taking their distinct origins, not, I 

 believe, the quite ancestral origin, but presenting a most 

 suggestive departure from that ancestral phase. To agree 

 with the ancestor, as in Zoophytes, the cells of the embryo 

 Holothurian, which are to give rise to muscles and skeletal 

 tissue, should originate from the whole internal surface of 

 the ectoderm or deron by delamination, but instead of doing 



Fi^.n 



Fix/. 16 



■ME 



Blji. Fid. 



Development of a Holothurian, after Selenka. 

 Blp. Blastopore ; Pid. Proctodseum ; Vpv. Vasoperitoneal or coelomic 

 vesicle, the Parenterou. M.E. Digestive Sac or Metenteron. 



this, they either proceed from the multiplication of a few 

 cells, separated at a very early period at the inner face of the 

 pole of invagination, or they originate a little later from that 



VOL. XVII. NEW SER. E E 



