118 MALL. [Vov. XTX. 
integrate as easily. The vascular system seems to suffer 
more than the nervous system does, but this may be due to 
the character of the primary trouble in the chorion, which 
probably first made itself felt in the heart. It is clear that 
when the heart is affected and stops that the embryo is 
then deprived of its nutrition, and under these circumstances 
the brain suffers before the spinal cord. Among the tissues 
the precartilages and cartilages suffer least of all. 
EMBRYOS OF THE SIXTH WEEK. 
In the beginning of the sixth week of development the 
cartilages of the extremities are outlined, and at the end of 
the week some of the ossification centers are present. Coinci- 
dentally the peripheral nerves ramify through the body and 
the muscle anlages appear. Thus we have before us a highly 
differentiated organism, and from now on anything which 
affects its nutrition does not produce a like influence in all 
of its tissues and organs. The reader has noticed, no doubt, 
that the present study is gradually leading in this direction. 
First the umbilical vesicle is most resistant, then the nervous 
system and now it is the skeleton. At first the blood-vessels 
possess the greatest power of growth before they were de- 
pendent upon the heart, but later when they are, they appear 
to suffer most, and the other structures are only affected in 
a secondary way, for they in turn receive their nutrition from 
the blood. 
The changes in embryos of the sixth week can be followed 
with greater ease than those in earlier embryos, for they are 
less rapid and there are a larger number of known structures 
present to tell the story. In studying the pathological em- 
bryos, I naturally compare these changes with the normal in 
embryos of about the same age, and as a standard Nos. 109 
and 144 are constantly employed (Plate IV, Fig. 10). 
