No. 408.] STUDY OF MAMMALIAN EMBRYOLOGY. 9I9 
important, interesting, and complicated developments by which 
the embryonic is changed into the adult anatomy. 
The body of the embryo shows the position and number of 
the segments (Seg.) by the external modeling. Both limbs are 
well advanced, the anterior (A.Z.) more so than the posterior. 
From the base of the front to the base of the hind limb extends 
the milk-line (47.Z.), curving so as to be nearly parallel to the 
dorsal outline of the body. Along this line the mammary 
glands are ultimately developed. Extending across the body 
are several shadowy lines, shimmering through the translucent 
body-walls. One marks the position of the embryonic dia- 
phragm ; it extends from the upper edge of the anterior limb 
obliquely downwards towards the edge of the umbilical cord. 
Another, which extends in a nearly straight line from limb to 
limb, marks the ventral edge of the large Wolffian body or 
mesonephros, the dorsal limit of which is approximately indi- 
cated by the milk-line (J7.L.). The outlines of the smaller 
left dorsal lobe of the liver are distinct, and mark out a pointed 
area immediately below the fore limb (A.Z.). 
Of the remaining figures, 2 to 7 represent six transverse 
sections through a pig embryo of 12.0 mm. But the position 
of the sections can be easily followed with the aid of Fig. 1, 
which represents a slightly younger embryo. The plane of 
the sections would be indicated by a nearly horizontal line 
across Fig. 1. The sections are ten microns in thickness, and 
are 966 in number, not 1200, as the student might expect. 
The discrepancy is due to the shrinkage of the embryo when 
imbedded in paraffine. The shrinkage is always very great, 
and in the case of embryos causes a loss of almost 20 per cent 
in the length ; but as it seems to take place uniformly through- 
out the embryo, it causes no distortion, so that the embryo in 
paraffine is an exact though greatly reduced copy — so to speak 
— of the living embryo. It should be remembered that no 
correct measurements of the size of organs or cells can be 
obtained from sections made by the paraffine method. This 
limitation upon the use of sections is too often forgotten. 
The six transverse sections chosen were selected to show 
the most important points in the structure of the embryo, as 
