56 



HUMAN ANATOMY. 



DEVELOPMENT OF THE GENERAL BODY-FORM. 



In considering the evolution of the external form of the human product of con- 

 ception, it is convenient to recognize the three developmental epochs suggested by 

 His, — the stage of the ovum, the stage of the embryo, and the stage of the foetus. 



The Stage of the Blastodermic Vesicle, — This stage, or the stage of the 

 ovum, embraces the first two weeks of intra-uterine life, during which the initial phases 

 of development, including fertilization, segmentation, and the formation of the blasto- 



FiG. 66. 



Embryo 

 with amnion 





Villous chorion 



Ectoblast 

 Mesoblast 



^  , -Belly-stalk 



Umbilical vesi- 

 cle; blood- 

 islands appear- 

 ing 



■■"^'Sfe 



''^g^ji^^i'^i^e^^g^^^^ifs^^^i:^^ 



Early human embryonic vesicle of about thirteen days laid open, showing the young embryo (.37 millimetre 

 long) attached to the wall of the serosa by means of the belly-stalk. X 25. (After Spee.) 



Chorionic 

 villus 



dermic vesicle, are completed, and the fundamental processes resulting in the differ- 

 entiation of the medullary tube, the notochord, the somites, and the,mesoblastic plates 

 are begun. The early details of many of these processes have never been observed 

 in man, but there is little reason to doubt that in its essential features the early human 

 embryo closely follows the changes directly observed in other mammals. 



The Stage of the Embryo. — The stage of the embryo, from the second to 



the fifth week, is distin- 

 FiG. 67. guished by the formation of 



organs essentially embry- 

 onic and transient in char- 

 acter, as the somites, the 

 notochord, the Wolfllian 

 body, and the visceral 

 arches. 



The earliest phase in 

 the differentiation of the 

 vertebrate body-form con- 

 sists in the establishment of 

 a dorsal tube by the appo- 

 sition and fusion of the ectoblastic medullary folds, and a ventral tube by the approxi- 

 mation and final union of the folds directly derived from the somatopleura. The 

 dorsal, or animal, tube represents the early neural canal, and becomes the great 

 cerebro-spinal nervous axis ; the ventral, or vegetative, tube, formed by the ventral 

 extension and approximation of the somatopleura, constitutes the body-cavity, and 

 encloses the primary gut and the associated thoracic and abdominal viscera, and the 

 vascular system. The primitive gut-tube originates by the delimitation of a part of 



Mesoblast 



Belly-stalk 



umbilical sac 

 Section of preceding embryonic vesicle and embryo. X 25. {After Spee.) 



