ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
591 
multiply in the meshes of the reticulum of adenoid tissue. In the 
second stage there are Malpighian corpuscles within which new leuco- 
blasts are formed by indirect division, and there is a splenic pulp which 
is reserved for giant-cells and erythroblasts which divide by mitosis. 
In the third or adult stage the pulp does not seem to take any part in 
the formation of new red blood-corpuscles, and there are no giant- 
cells. 
The osseous medulla of Mammals diffors much from that of Birds ; 
in the latter the venous capillaries have a continuous endothelial wall ; 
in consequence of this the blood circulation does not pass through them, 
and the erythroblasts are developed and multiply only within the blood- 
forming capillaries. The various kinds of white cells pass by diapedesis 
through the endothelial wall, and multiply outside the vascular plexus. 
In Mammals the walls of the venous capillaries are discontinuous ; the 
blood passes by the opening, and penetrates freely into the meshes of 
the adjacent adenoid tissue ; within the plexuses the erythroblasts 
and leucoblasts divide and develope. 
Studies on the Blood of Amphibia.* — In this memoir Dr. A. B. 
Macallum deals with the origin of haemoglobin, the fusiform corpuscles, 
and the origin of the heematoblasts. As to the first point he finds that 
the haemoglobin of the blood-corpuscles is derived from the abundant 
nuclear chromatin of the hsematoblast. The fusiform cells of Amphibian 
blood are derived from the red corpuscles, which lose their cell-mem- 
brane, and the greater portion of the discoplasma. The haematoblasts of 
Amblystoma are direct descendants of cells split off from the extreme 
ventral portions of the visceral mesoblast, and they pass, at first, a 
portion of their existence in a specialized part of the original body- 
cavity of the embryo. 
Origin and Evolution of Mammalian Teeth. | — Prof. W. Kiikenthal 
discusses this difficult question. Some think the milk teeth a secondary 
acquisition, others regard them as primary, and Baume suggests that 
both sets are secondary, the original dentition being regarded as a 
homodont reptilian-like series, of wbich some members are suppressed 
with the shortening of the jaws, while others are modified to appear sub- 
sequently as the final set. The fact is, that the succession of teeth in 
Mammals has not yet been explained. Kiikenthal starts from the teeth 
of toothed whales, which in most cases form a uniform homodont series, 
but even apart from the narwhal, the series is not always quite homodont, 
as an examination of the dentition of embryos of Phocsena communis 
shows. It is generally believed that these teeth represent the second 
set. Weber, Julin, and Winge have, with slight differences, sup- 
posed that in association with the long jaws both sets of teeth appear 
together and are indistinguishable. But Kiikenthal maintains that the 
teeth of toothed whales represent a true milk dentition. He bases his 
conclusion on the fact that the second set is present in rudiment in 
embryonic life, but afterwards disappears. 
This fact leads him to reject all hypotheses according to which the 
toothed Cetaceans are primitive homodont and monophyodont animals, 
* Trans. Canadian Institute, ii. (1892) pp. 221-60 (1 pi ). 
f Jenaische Zeitschr. f. Naturwiss , xxvi. (1892) pp. 469-89. 
