I30 WHITMAN. [Vol. I. 



similarity extends to the oesophageal portion of the alimentary- 

 canal. 



It is an interesting fact that in those cases where the major 

 portion of the mesenteron passes through well-marked stages of 

 differentiation, beginning with one or more primary vitelline 

 spheres, then, with or without further subdivision of these 

 spheres, taking the form of free peripheral nuclei imbedded in 

 protoplasm with no defined cell boundary-lines (entoplasts), 

 and finally assuming the form of a distinct lining epithelium ; 

 there is always an anterior portion, which arises quite early, and 

 which, from the first, consists of distinct cells, often scarcely 

 distinguishable from the cells of the ectoderm and mesoderm. 

 Numerous cases might be cited, but I must here limit the com- 

 parison to Rhynchelmis. On this point I may refer again to 

 Vejdovsky. Speaking of the origin of the entoderm at the an- 

 terior end of the embryo, he makes the following remarks : — 



" Es bildet sich hier eine Gruppe dicht neben und anein- 

 ander liegenden Hypoblastzellen, die der DotterkilgelcJien v'dllig 

 entbeJiren und sich ebenso intensiv roth wie die Keimstreifsellen 

 tingiren. Den ersten Ajifajig derartiger Hypoblastzellen kann 

 man bereits in dem Stadium Jinden, als die Mesomeren sich 

 eifisustillpen ^ und die ersten Keimstreifzellen zu produciren 

 bcginnen. Aus den derart differencirten Hypoblastzellen bildet 

 sich sp'dter die Epithelschicht des Anfangstheiles vom Mittel- 

 darme — der Oesophagus. (No. 14, p. 237). 



Branchiobdella. — In a recent paper on the embryology of 

 Branchiobdella, Salensky (No. 15, p. i) has compared this form 

 with Clepsine and Nephelis. The paper is a most welcome ad- 

 dition to our knowledge of this interesting parasite ; but the 

 points of chief interest here — the cleavage, axial relations, and 

 origin of the germ-layers — are too imperfectly worked out to 

 admit of a close comparison with Clepsine. 



Salensky finds on one pole of the egg a clear spot, which 



' The " Einstulpung " of the mesomeres, described by Vejdovsky, is comparable 

 with the movements which result in imbedding the teloblasts of Clepsine in the vitel- 

 line spheres. I believe this imbedding process is best understood as a part of the 

 invaginatory movement described in my former paper (No. i, pp. 58-59). The 

 mesoblasts lie at the hind end of the blastopore, betweeft the entoblast and the ecto- 

 blast, and their products are carried inward between the two primary layers. 



(15.) Salensky, W. Developpement de Branchiobdella. Arch, de Biol. vi. 

 Fasc. I, p. I. 1885. 



