1890.] The Mesoderm and the Calom. 893 
egg protoplasm, which has its cell division (segmentation) re- 
tarded, and the cells, whether early or tardily produced, immi- 
grate into and between the germ-layers already developed. 
The opposition to the parablast theory is the sum of numerous 
observations which, as pointed out in the previous part of this 
chapter, prove, it seems to me, that the mesoderm arises in all 
vertebrates (except mammals ?) as a unit, and subsequently separ- 
ates into mesothelium and mesenchyma. The leading opponent of 
the separate oHG of the Te is Kölliker, in both his text- 
books (Ent ichte, etc., and Grundriss), and in separ- 
ate articles (see especially 19,20, and his criticism, 27,of Kollmann). 
Iagree with Kolliker that it has been sufficiently demonstrated that 
the * acroblast" belongs to the entoderm, and that after delamina- 
tion of the mesoderm the acroblast is transformed into the epi- 
thelium of the yolk-sac ; for a conclusive demonstration that this 
is so in reptiles, see H. Strahl, 4o. 
The cælom theory of the brothers Hertwig includes a funda- 
mental modification of the parablast theory. The main features 
of the coelom theory are not original with the Hertwigs, but may 
be found in previous writers. Nevertheless, they were the first 
to present the theory in a complete formula, and with a backing 
of facts, both new and collected from others, so extensive as to 
compel attention. In justice to E. Ray Lankester it must be 
stated that he is really the author of the ccelom theory, having, 
in 1877 (27, A), published the hypothesis that the coelom is de- 
rived from the archenteron, and that the mesoderm of vertebrates 
represents solid entodermal diverticula. It is unfortunate that 
the Hertwigs have not made due acknowledgment of what they 
owed to Lankester and others. They made a series of investiga- 
tions on the germ-layers of various representatives of the animal 
kingdom, and presented their general results in a comprehensive 
article (O. and R. Hertwig, 74). O. Hertwig has again ex- 
pounded the theory in his text-book of embryology. The 
coelom theory consists of two parts: 1°, the cælom is formed by 
diverticula of the archenteron and its lining ; the mesothelium is 
part of the entoderm; 2°, the mesenchyma consists of cells 
thrown off by the germ-layers, and is essentially distinct from the 
Am. Nat.—October.—2. 
