No. 408.] STUDY OF MAMMALIAN EMBRYOLOGY. 94I 
from the embryonic mesenchyma. The simplicity of mesen- 
chymal structure is further emphasized by the absence of dis- 
tinct layers. Thus, between the brain and the ectoderm there 
is a continuous layer of tissue nearly uniform in appearance ; 
later, out of this layer there will have arisen the cutis, subcuta- 
neous tissue, periosteum, bone, dura mater, arachnoid, and pia 
mater, while the complexity of organization will have been 
increased by the ingrowth of nerves and striated muscles. The 
entoderm and ectodermal epidermis are both quite simple epi- 
thelial structures, although the entoderm, it must be added, 
already presents some very important modifications, such as 
the notochord, the thymus and thyroid glands, and the liver. 
On the other hand, the neuro-ectoderm shows advanced differ- 
entiation of the neurones with their accompanying axons, from 
the neuroglia cells. The mesothelium also offers more com- 
plexity, owing to its share in the development of the striated 
muscles, of the Wolffian body, and of the genital glands. 
The preceding illustrations and descriptions are published, 
partly in the hope that in spite of their fragmentary character 
they will prove useful to some students of mammalian embry- 
ology, partly in the hope that the author will receive from 
other teachers criticism and suggestions in regard to the 
proposed text-book. 
HARVARD MEDICAL SCHOOL, BosTON, Mass. 
October 23, 1900. 
