1880. | A Sketch of Comparative Embryology. 877 
sels, and in vertebrates the lymphatic system. Another set of 
cavities forms the excretory system—the water vessels (of certain 
worms), the segmental organs and kidneys, all distinguished by 
being connected directly with the exterior by openings through 
the ectoderm. There are also tubular ducts which compose the 
secondary genital apparatus, and are, in many of the higher inver- 
tebrates and in all vertebrates, intimately connected with the 
excretory organs. Formerly it was supposed that the branching 
respiratory tubes or frachee of insects, were mesodermic, but 
more recent investigations tend to show that they are always 
invaginations of the ectoderm. All these cavities are lined each 
by a layer of cells, one row deep, an epithelium. In the circula- 
tory channels and body-cavity, the epithelium appears to be inva- 
riably composed of broad, irregularly polygonal very thin cells, 
being a so-called pavement epithelium, while in the excretory 
tubes and genital ducts the epithelium is quite thick, each cell 
being at least as high as it is broad. 
The seventh law is of the utmost importance—each germinal 
layer forms predetermined special tissues, and no others, and 
each tissue in a predetermined position. In all bilateral animals 
at least, the mesoderm forms, besides the organs belonging to it 
exclusively, such as the heart, etc., layers of tissue around the 
whole entoderm and ectoderm ; for example, the intestine of an 
adult animal is composed of an entodermal lining (epithelium) 
and. several mesodermic coats (connective tissue and muscles) ; 
the skin is composed of an outside epidermis, derived from the 
ectoderm, and under it the dermis, or cutis, derived from the 
mesoderm, An organ is said to be ectodermal or entodermal 
when the part essential to its physiological function arises from 
one or the other of the primitive layers; for example, the eye is 
ectodermal because its light perceiving portion is developed from 
the outer germ layer; the liver on the other hand is entodermal 
because its secreting cells are formed from the inner germ layer. 
The anatomy of adult forms does not by any means always 
reveal to which layer a given organ properly belongs, This is 
perhaps better illustrated by the nervous system than by any 
l There are certain exceptions, e. g., the malpighian bodies of the vertebrate kid- 
neys are lined by a pavement epithelium although they form part of the excretory 
System of cavities. 
* Often called Aysodermis by many writers on Invertebrates, especially by ento- 
mologists, ' 
