No. 408.] STUDY OF MAMMALIAN EMBRYOLOGY. 915 
Fifth, a study of the genital elements, impregnation, seg- 
mentation, etc. 
This scheme is not logical in its order, but it seems to me to 
follow that psychological order which is natural for beginners 
in embryology who have already studied anatomy. It is, I 
believe, often a mistake in introducing a pupil to a new sub- 
ject to sacrifice the natural, and therefore easy, psychological 
sequence to the artificial demands of a logical arrangement. I 
hope, therefore, that the scheme adopted will prove advanta- 
geous for laboratory work. | 
Most stress will be laid upon the young mammalian embryo. 
Figures have been prepared to show several stages of the pig, 
all magnified eight diameters. To represent the first stage to 
be studied, a pig of 10.0 mm. has been chosen (Fig. 1). This 
figure, as well as the others in this article, are woodcuts after 
pencil drawings. The drawings were made for me by Mr. E. A. 
Locke with great care and accuracy. The woodcuts were made 
by Mr. C. L. Albert Probst, of Braunschweig, Germany, and 
reproduce the original drawings with remarkable and satisfac- 
tory success. The German school of wood engraving uses the 
black line, while the American school uses the white; so that 
a German woodcut reproduces microscopical textures better 
than an American, since in our s/azzed sections we rely in our 
studies chiefly upon the dark or stained parts, and these are 
indicated by the black-line better than by the white-line engrav- 
ing. There is the further advantage that the cost in Germany 
is considerably less. The wood engravings combine softness 
and delicacy of tone with brilliancy of texture, and thus sur- 
pass, it seems to me, very considerably even the best “ process” 
engravings. As the number of good figures of sections of 
mammalian embryos is very limited, and as there is no series 
of illustrations of systematically chosen typical sections of 
typical mammalian embryos, it is hoped that the present series 
will be useful, not only to students, but also as a conven- 
ience to advanced workers. On the other hand, it cannot 
be claimed that the new pictures represent definite original 
research, because they are intended primarily for text-book 
use. I have pleasure in acknowledging very valuable assistance 
