ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
5S9 
more than a long narrow loop of a capillary of wide bore with a wall 
one cell thick, enclosed in a folded sheet, also only one cell thick, of 
small epithelial cells which consist of little but a great nucleus. These 
filaments enfold the yolk-sac and seem to assist in absorbiug the 
nutrient yolk. 
After reminding us that the modes of reproduction among Elasmo- 
branchs are three — (1) oviparity, (2) viviparity with the formation of 
a placenta, and (3) viviparity without the formation of a placenta, Dr. 
Alcock offers some suggestions as to the origin of a placental vivi- 
parity among Elasmobranchs, in which he expresses the belief that the 
second mode arose directly from the first of these, and the third from 
the second. 
Mesoderm of Teleostean Fishes.* — Mr. E. R. Boyer has especially 
studied (in the Cyprinodont Fundulus heteroclitus) the share taken by 
the mesoderm in the formation of the pectoral fin. He first, however, 
describes the formation of the primitive layers and the differentiation 
of the protovertebrae and lateral plates, the differentiation and early 
development of the mesoderm in the pectoral region, and the origin 
and fate of the “ intermediate cell-mass.” With regard to the origin 
of the pectoral fin the first step is a differentiation in the somatopleure 
caused by cell-proliferation in the region of the nephrostome. This 
process leads to the formation of the pectoral plate. Between this last 
and the four most anterior myotomes a connection is formed, and 
elements from the peripheral layer of these myotomes are contributed 
to the pectoral plate. The head-mesoderm does not really appear to 
be concerned in the earliest formation of the pectoral fin. 
The ectodermal fold of Fundulus does not begin to be formed 
until several days after the formation of the pectoral plate in the 
mesoderm ; there is not in this Fish any trace of a continuous longi- 
tudinal modification of the ectoderm along the side of the embryo, 
such as has been observed by Balfour and by Ryder. 
Formation of Germinal Layers of Petromyzon. f — From his study 
of Petromyzon , Mr. S. Hatta concludes that the formation of the 
germinal layers accords well with the general plan of the development of 
these layers in Vertebrates, as shown in Amphioxus , Triton , and Bana , 
Trionyx and Clemmys , and it is, therefore, unnecessary for us to enter 
into detail. Calberla, Scott, Shipley, and Kuppfer have already studied 
the subject, and it is with the first of these that Mr. Hatta’s results 
mainly agree. 
Development of Blood-corpuscles. :£ — Dr. O. Van der Stricht finds 
that the first blood-corpuscles appear at the level of the area vasculosa 
and at the expense of mesoblastic elements. They all present the 
characters of red corpuscles. The white cells appear later in the 
circulation, and also arise from mesoblastic elements formed outside 
the capillaries. From the time of their appearance the red and white 
corpuscles are distinct in structure and origin. 
The multiplication of blood-cells already formed continues in the 
* Bull. Mus. Comp. Zool., xxiii. (1892) pp. 91-134 (8 pis.). 
t Journal Coll. Sci. Imp. Univ. Japan, v. (1892) pp. 129-47 (2 pis.). 
X Arch, de Biol., xii. (1892) pp. 199-344 (6 pis.). 
