58o THE POPULAR SCIENCE MONTHLY. 



of nutrition, there must result an increased power of development and 

 growth. 



By the absorption of fluid from the maternal tissues in which it is 

 imbedded and the accumulation of this fluid at the center of the mass, 

 the cells of this mulberry-like body become crowded outward to the 

 periphery, thus forming a lining for the membranous sac — i. e., the 

 outer covering of the ovum — which incloses them, the entire globular 

 mass now being about one twenty-fourth of an inch in diameter, and 

 consisting of a structureless outer membrane lined with a layer of nu- 

 cleated cells (the hlastoderm), and filled with clear fluid. These lining 

 cells multiply rapidly ; the inner ones become larger, darker, and 

 softer than the outer ones, and thus differentiation has again occurred 

 — the lining having developed into two distinct layers. This is known 

 as the gastrula stage of embryonic development. All animals, from 

 sponges to man, pass through this phase, becoming first two and then 

 three layered sacs ; but, from this point, the different branches or sub- 

 kingdoms diverge ; and the next recognizable phase in the develop- 

 ment of the human embryo is confined to vertebrates, with a single ex- 

 ception, the ascidian. The larval ascidian swims like a tadpole by 

 means of a caudal appendage in which may be traced a rod-like body 

 thought to be a rudimentary chorda dorsalis, since it resembles the 

 embryonic structure which, in the perfect vertebrate, develops into the 

 spinal column with its contained, highly endowed spinal cord. This, 

 however, not only fails to develop but actually disappears in adult 

 life, leaving the ascidian a simple invertebrate animal. But, whether 

 the ascidian be a true connecting link between invertebrates and ver- 

 tebrates, or, as suggested by Balfour, a reversion from the higher 

 form, it serves equally to indicate a close relationship between these 

 two great subdivisions of the animal kingdom. 



Between the two layers of germinal cells which belong to the gas- 

 trula stage, a third layer is developed, and from these three layers (the 

 epiblast, the mesohlast^ and the hypoblast) all the tissues and organs of 

 the body are derived. The inner layer (hypoblast) gives origin to the 

 epithelial lining of the alimentary canal and to the various glands de- 

 rived from it. From the outer layer (epiblast) are developed the brain 

 and spinal cord, and the epidermis with its appendages and derivatives, 

 including the organs of the special senses. From the middle layer 

 (mesoblast) the various intermediate structures are produced. The 

 remaining history of development is, therefore, the history of the dif- 

 ferentiation of these three layers of the blastoderm (which alike con- 

 sist of simple nucleated cells) into the various tissues and organs of 

 the body. Accompanying this process there is a corresponding de- 

 velopment of functions. As absorption and assimilation, so perfectly 

 performed by these germinal cells, are, however, the fundamental facts 

 in the nutrition of even the highest organisms, so also reaction in re- 

 sponse to a stimulus, of which we have found even the moner and the 



