298 C. O. WHITMAN. 



mesoderm arises from a single pair of cells, placed at the posterior 

 end of the blastopore, there are others which are more or less 

 doubtful. Biitschli ( ., ^V-h ) ^^^o ^'^^ ^^^ ^^^^ to devote special 

 attention to the germ-lamellse in Nephelis, found tlie mesoderm 

 at first as two lateral lines of cells. Whether they were produced 

 by two primary mesoblasts, was not ascertained. Kowalevsky 

 (V) did not succeed in tracing the mesoderm in the Hirudinen. 

 InEuaxes (-f^) the mesoderm was referred, in part at least, to two 

 cells. Whether these two cells were'really mesoblasts or neuroblasts, 

 admits of doubt. It seems probable, though not certain, that the 

 mesoderm has a similar double origin in ISTassa (Bobretzky, tt'v)- 

 Bobretzky derived the mesoderm from the ectoderm. Kabl ( , ,', '1% ) 

 found a double symmetrical arrangement of the mesoderm in Lym- 

 nreus, and derived it from entoderm and ectoderm. According to 

 BiitschH (-^^i wtt) the mesoderm appears as a few cells (pro- 

 bably bilaterally symmetrical), at the hind end of the blastopore 

 in Paludina. Biitschli thinks they are of entodermic origin, 

 while Lankester (ytto-) refers the origin to both ectoderm and 

 entoderm. Langerhans (tWIiVt^~s") found in several Gastero- 

 pods (Accra, Doris, Aeolisj i/co large cells at ihe hind end of the 

 blastoporic cleft, but was able to give no explanation of the same. 

 Biitschli [Jf-^, Neretina j mentions a pair of cells {x) as arising 

 from the large blastomeres (see fig. -16, PI. XVII), the fate of 

 which was unknown. The same may be said of a pair of cells 

 found in the egg of Helix (78, fig. 5). These doubtful cases are 

 mentioned only in the hope of turning the attention of embryo- 

 logists more to this point. 



Indications of this double origiu are not wanting in other 

 classes of animals. In Sagitta (Kowalevsky, ^VV) the mesoderm 

 arises as lateral diverticula of the entoderm.^ Kowalevsky, in a 

 very valuable paper (-, ,", ;". /), has demonstrated the same thing 

 for Amphioxus, and at the same time proved the correctness of 

 Balfour's explanation {^^) of a fact noted in the development 

 of the Selachians (j-Vr) — namely, that the body- cavity extends 

 originally to the top of the protovertebra?. The first to suggest 

 that the mesoderm, viewed phylogenetically, arose as paired out- 

 growths of the entoderm, was Lankester. Commenting on Pro- 

 fessor Huxley's view of the body-cavity (this Journal, January, 

 1875), Lankester says : " I wish now very brieily to point out 

 that viewing the matter genealogically it is quite possible that by 

 the obliteration of the lumen of gastro- vascular outgrowths of 

 the jirimitive alimentary canal a large bulk of cellular elements 

 should be furnished to the so-called ' mesoblast ' from the hypo- 

 blast, and that subsequently this solid mass of cellular elements 



' In Plcurobranchidium, according to Lankester, a part of tlic cutodcnn 

 lies outside the alimentary cavity, and suggests a comparison witii the de- 

 velopment of Sagitta. 



