916 THE AMERICAN NATURALIST. [Vor. XXXIV. 
in the identification of parts rendered by Drs. R. T. Atkinson 
and F. A. Woods, and by Mr. F. T. Lewis and Mr. J. L. 
Bremer. Mr. Lewis has made also several dissections of the 
cephalic nerves, and Mr. Bremer models by Born's method of 
the brain and pharynx. I hope the results of these researches 
will be published separately. 
I wish also to express my appreciation of the great excellence 
of the recent text-books of embryology which have appeared in 
England, France, and Germany, although I venture to point 
out that they differ in plan from the proposed new book. 
A mammalian embryo may be conveniently regarded as 
having assumed its typical class organization at the time when 
the limb-buds have just become distinct appendages, as found 
in pig embryos of 9 to 12 mm. At this stage a mammalian 
embryo is readily distinguished from that of any other class of 
vertebrates, and the differentiation of the anlages! of all the 
important organs is accomplished so that these anlages can be 
identified with certainty and their genetic relations to the adult 
structures can be clearly grasped by the student. At the same 
time, although the anatomical differentiation is well advanced, 
the histological differentiation has made very little progress, so 
that the stages in question are particularly instructive to begin- 
ners. Fig. 1 represents a pig embryo of 10 mm. The student 
should make a careful and thorough study of the external form, 
noting the following features. The head, which is very large 
in comparison with the body, forms as a whole nearly a right 
angle with the back, so that dorsal outline of the head forms 
a distinct though rounded angle with that of the back; this 
angle marks the position of the neck-bend, and corresponds to 
the junction of the brain with the spinal cord. The neck-bend 
is one of the most marked and distinctive characteristics of 
the mammalian embryo, being much less marked in birds and 
reptiles, and being absent in amphibians and fishes. It 1s 
probably closely correlated with the cramping of the ventral 
cervical region, which leads to the formation of the cervical 
1 Anlage is from the German, and has been adopted as a technical term 2 
designate the first accumulation of formative material recognizable as the 
commencement of an organ, structure, or part in a developing organism. 
