The development of the human body; a manual of human embryology . omic space later , as has already been pointed out, the layerof enveloping cells splits into two concentric layers, theinner of which seems to be mesodermal in its nature andforms a layer lining the interior of the trophoblast andlying between this and the primary endoderm. Thislayer is by no means so evident in the lower forms, but is 78 THE DEVELOPMENT OF THE HUMAN BODY. perhaps represented in the reptilian ovum by the cellswhich underlie the ectoderm in the regions peripheral tothe blastoderm proper (see p.
The development of the human body; a manual of human embryology . omic space later , as has already been pointed out, the layerof enveloping cells splits into two concentric layers, theinner of which seems to be mesodermal in its nature andforms a layer lining the interior of the trophoblast andlying between this and the primary endoderm. Thislayer is by no means so evident in the lower forms, but is 78 THE DEVELOPMENT OF THE HUMAN BODY. perhaps represented in the reptilian ovum by the cellswhich underlie the ectoderm in the regions peripheral tothe blastoderm proper (see p. 70). The Significance of the Germ Layers.—The formation ofthe three germ layers is a process of fundamental impor-tance, since it is a differentiation of the cell units of theovum into tissues which have definite tasks to fulfil. Ashas been seen, the first stage in the development of thelayers is the formation of the ectoderm and endoderm, or,if the physiological nature of the layers be considered, itis the differentiation of a layer, the endoderm, which has. Fig. 34.—Transverse Section through the Embryonic Disk of a , Chorda endoderm; ee, ectoderm; en, endoderm; gin, gastral meso-derm.—{After van Beneden.) principally nutritive functions. In certain of the lowerinvertebrates, the class Coelentera, the differentiation doesnot proceed beyond this diploblastic stage, but in allhigher forms the intermediate layer is also developed, andwith its appearance a further division of the functions ofthe organism supervenes, the ectoderm, situated upon theoutside of the body, assuming the relational functions, theendoderm becoming still more exclusively nutritive, whilethe remaining functions, supportive, excretory, loco-motor, reproductive, etc., are assumed by the manifold adaptations of development obscure in THE SIGNIFICANCE OF THE GERM LAYERS. 79 certain cases the fundamental relations of the three layers,certain portions of the mesoderm, for instance, fa
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