172 The American Naturalist. [March, 
than the right. The author asks, is this embryonic asym- 
metry peculiar to man ? And also, does it bear any relation 
to the functional predominance of the right side over the left 
in the adult. This memoir is a valuable one, as it supplies a 
thorough study of one very important stage of the human em- 
bryo, and is a very creditable continuation of the work of His 
and Fol in the same direction. 
On the Development and First Traces of the An- 
terior Roots of the Spinal Nerves in Selachians.' — 
This last of Prof. Dohrn's studies forms chapter xiv. of the 
Studien zur Urgeschichte des Wirbelthierkbrpers ; it is most 
suggestive as is all of his work. The problem of the origin of 
the anterior or motor roots of the spinal nerves has given rise 
to a great deal of speculation and discussion. It has been the 
good fortune of Dohrn to find in embryos of Miistelus and 
Pristiurus 3 mill. 5.5 mill, and 10 mill, long, conditions of the 
development of the anterior or motor roots which are of great 
importance. 
1. The motor roots grow out at the lower angles of the med- 
ullary tube before the appearance of the white matter of the 
cord as conical or more or less produced extensions of the 
plasma of that tube. At first these roots contain absolutely no 
nuclei, but are simply homogeneous pseudopod-like processes. 
2. Mesodermal cells next approach and sink into these plas- 
mic processes. These probably have something to do with the 
development of the primitive sheaths of the future nerve fibres. 
3. These plasmatic ventral processes from the medullary 
tube now blend over the extent of their outer surfaces with the 
still undifferentiated plasma of the adjacent cells of the proto- 
vertebrae or somites. Junction of the motor portion of the 
nervous mechanism with the tissue still to be converted into 
■muscle is thus found to have taken place before even the for- 
mation of true nerve fibres or of muscular fibrillce. 
4. The next step in the differentiation of the motor roots is 
the migration of medullary cells into the above mentioned 
plasmatic processes from within the walls of the medullary 
tube. This seems to be conclusively established by the fact 
that the nuclei of medullary cells were seen in process of 
division at or within the bases of these processes. 
It seems to be thus conclusively established that of the prim- 
lUeber die erste Anlage und Entwickelung der motorischen Riickenmarks 
nerven bei den Selachiern. Mitth. aus d Zool. Stat, zu Neapel. viu. 1888. pp. 
441-461. Taf. 22. 
