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THE POPULAR SCIENCE MONTHLY 



differs in different classes of animals to such an extent that it is diffi- 

 cult to formulate any general description which will apply to all of 

 them. Usually the many cleavage cells form a hollow sphere, the 

 blastula (Fig. 10, E), and this in turn becomes a gastrula (Fig. 10, L, M), 

 in which at first two, and later three, groups or layers of cells may be 

 recognized; the outer layer, which is formed from cells nearest the 

 animal pole of the egg, is the ectoderm; the inner layer, or entoderm, is 

 formed from cells nearest the vegetative pole; a middle layer, or group 

 of cells, the mesoderm, is formed from cleavage cells which in vertebrates 

 lie between the animal and vegetative poles. 



5. Organogeny. — By further differentiation of the cells of these lay- 

 ers and by dissimilar growth and folding of the layers themselves 

 the various organs of the embryo begin to appear. From the ectoderm 

 is formed the outer layer of the skin and the nervous system; from the 

 entoderm arises the lining of the alimentary canal and its outgrowths ; 

 from the mesoderm comes, in whole or in part, the skeletal, muscular, 

 vascular, excretory, and reproductive systems. In vertebrates the ner- 

 vous system appears as a plate of rather large ectoderm cells (Fig. 12) ; 



vies 



coel 



som 



Fig. 12. Ckoss Section* of Amphioxus Larvae in Successive Stages of De- 

 velopment. A, through a larva similar to 11A ; B and C, of a larva similar to 11B ; 

 D, of a still older larva ; ect, ectoderm ; ent, entoderm ; mes, mesoderm ; ch, noto- 

 chord ; np, neural plate ; gc, gastroccel ; ac, alimentary canal ; coel, ccelom. 



this plate rolls up at its sides to form a groove (Fig. 12) and then a tube 

 (Fig. 12) ; and by enlargement of certain portions of this tube and by 

 foldings and thickenings of its walls the brain and spinal cord are 

 formed (Figs. 12, 15, C, D). The retina or sensory portion of the eye is 

 formed as an outgrowth from the fore part of the brain (Fig. 15, D) ; 

 the sensory portion of the ear comes from a cup-shaped depression of the 

 superficial ectoderm which covers the hinder portion of the head 

 (Fig. 15, E and F). The back bone begins to appear as a delicate cellu- 



