9I4 THE AMERICAN NATURALIST. [Vor. XXXIV. 
of the chick, — the one chiefly, the other exclusively, — and 
furnish little aid for mammalian embryology. Marshall’s 
Embryology is on a more comprehensive plan, but as it scarcely 
passes beyond the general principles of vertebrate embryology 
it cannot satisfy the needs of anatomical study. There is 
in these three works the merit of an adaptation to practical 
laboratory study, but there are no recent manuals of this type. 
On the other hand, there is, as all morphologists know, an 
abundance of recent embryological text-books ; but all of them, 
I think without exception, follow the general pattern established 
by Kölliker in the first edition of his Extwickelungsgeschichte 
(1861), and offer generalized descriptions of the development of 
the germ-layers, and then of the various organs. A beginner 
profits more from a presentation of the subject which clings to 
the actual preparations which he may make for himself. These 
considerations have led me to the conviction that an introduc- 
tory work more or less upon the model of Foster and Balfour 
would be useful. It is such an * Introduction" which I have 
undertaken. 
The plan adopted is briefly as follows : 
First, a study of pig embryos of from 9 to 12 mm. Experi- 
ence has shown that embryos of these sizes can be obtained 
in considerable numbers from the pork-packing establishments, 
such as may be found near most large cities. The anatomy of 
the pig at this stage is readily understood by the student who 
knows the general anatomy of the adult. Older embryos are 
more complicated, and yield such long series of sections that 
the beginner is apt to be discouraged ; younger embryos, owing 
to their spiral twisting, are exceedingly difficult for students to 
understand when sectioned. 
Second, a study of pig embryos of about 17.0 mm. and of the 
head of an embryo of 20 to 24 mm., which will suffice to render 
clear the relation of the early embryonic anatomy to the adult 
structure. 
Third, a study of the chick embryo of 20 to 30 hours to 
illustrate the character of the germ-layers. 
Fourth, a study of the fcetal envelopes, including the 
placenta in man and perhaps also in the pig and rabbit. 
