878 The American Naturalist. [October, 
I. ORIGIN OF THE MESODERM. 
Mesoderm of Elasmobranchs —In the cartilaginous fishes the 
mesoderm arises from the.entoderm close to the ectental line. 
The observations of Balfour in his monograph, 2 (see also his 
works, I., 246-268), established the fact that the mesoderm 
appears after the two primary layers and is connected with the 
entoderm. This fact has since been abundantly confirmed (see 
Kollmann, 25; Swaen, 47; Rickert, 32, 33; Rab, 29; D 
Schwarz, 36, et al). These later observations, particularly those 
of Rückert and Rabl, have settled the exact point, or rather area, 
of entoderm which is mesoblastogenic. Unfortunately Rabl over- 
looked the phenomena of concrescence, and consequently reached 
conclusions as to the development of the mesoderm which I feel 
no hesitation in pronouncing erroneous. The mesoderm is differ- 
entiated along the embryonic rim before concrescence takes place ; 
hence, when concrescence is partly completed, there is an axial 
stretch of mesoderm, and from the hind end of this the mesoderm 
spreads out toward each side along the embryonic rim in con- 
nection with the entoderm, as has been described! We can dis- 
tinguish the axial mesoderm from the lateral mesoderm; but 
later on, when concrescence has progressed further, there is no 
lateral mesoderm, for it has become axial. Rabl, however, 
failed to study the later stages, and so came to consider that this 
temporary condition of the mesoderm signified a double origin ; 
accordingly he distinguishes between the “gastral” (axial) and 
* peristomal " (lateral) mesoderm, and makes the unsuccessful 
attempt to show that the “gastral " and “peristomal” mesoderms 
are of essentially different origin in all vertebrates. Had Rabl 
accepted the law of concrescence, he would certainly have not 
fallen into these errors. There is no evidence whatever that 
there is an evagination of the entoderm, as the Hertwigs maintain 
can be shown in the amphibians (see below). On the contrary, 
the cells grow forth from the entoderm, so as to constitute a 
sheet between the primary germ-layers. Soon the connection 
with the entoderm is permanently severed. 
! AMERICAN NATURALIST, 1890, p. 507. 
