MarcH 29, 1901. ] 
their professional study even in this one 
direction ? E 
The accompanying table presents the 
principal tissues classified according to 
their layership. There have been classi- 
fications of organs on the layership basis 
published before, but inasmuch as organs 
usually contain cells from two layers, we 
get a more correct presentation of the ac- 
tual genetic relationships by restricting our 
tabulation to the tissues. Leucocytes do 
not appear in the table for the reason that 
SCLENCE. 
487 
erto been clearly recognized or defined. For 
both types the starting point is the same, 
the undifferentiated embryonic cell. In 
one type we find that as the cells proliferate, 
a portion of them only undergoes differentia- 
tion, and another portion remains more or 
less undifferentiated and retains more or 
less fully the power of continued prolifera- 
tion. The epidermis is a good representa- 
tive of this type. Its basal layer consists 
of embryonic cells, which multiply ; some 
of these cells move into the upper layers, 
CLASSIFICATION OF THE TISSUES. 
A. ECTODERMAL. 
1. Epidermis. 1. Mesothelium. 
a. Epidermal appendages. 
B. MESODERMAL. 
a. Epithelium of 
C. ENTODERMAL. 
1. Notochord. 
2. Epithelium of 
b. Lens of eye. peritoneum, a. Digestive tract, 
2. Epithelium of pericardium, esophagus, 
a. cornea. pleura, stomach, 
b. olfactory chamber. urogenital organs. liver, 
ce. auditory organ. b. Striated muscles. pancreas, 
d. mouth 2. Mesenchyma. small intestine, 
(oral glands), a. Connective tissue, yolk-sack, 
(enamel organ), Smooth muscle, large intestine, 
(hypophysis). Pseudo-endothelium, cecum, 
e. anus. : Fat-cells, vermix, 
f. chorion, Pigment cells. rectum, 
Foetal placenta. b. Blood. Allantois (bladder). 
g. amnion. c. Blood vessels. b. Pharynx, 
3. Nervous system. d. Lymphatics. Eustachian tube, 
a. Brain, e. Spleen. " Tonsils, 
optic nerve, f. Supporting tissues, Thymus, 
retina. cartilage, Parathyroids, 
b. Spinal cord. bone. Thyroid. 
ce. Ganglia. g. Marrow. c. Respiratory tract, 
d. Neuraxons. Larynx, 
Trachea, 
Lungs. 
their first origin is uncertain. Blood cells 
arise very early, before the clear separation 
of mesoderm and entoderm has occurred— 
it is possible that they are entodermal. 
With these two limitations, the table pre- 
sents our present knowledge. 
We will now turn to the analysis of the 
differential process in each germ layer. 
We have to deal with changes in cells. 
There are two distinct types of cell dif- 
ferentiation, which I think have not hith- 
enlarge and differentiate themselves into 
horny cells; others remain in the basal 
layer and continue to multiply. The prog- 
eny of a given basal epidermal cell do not 
all have the same fate, but divide them- 
selves into two kinds of cells, one kind re- 
taining the ancestral character, the other 
becoming something new and unlike the 
parent cell. Differentiation according to 
the second type is characterized by its in- 
clusion of all the cells. This type has its 
