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CHAPTER X. 

 ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. 



AFTER having investigated in the preceding chapters the fundamental 

 organs of the body of vertebrated animals, or the germ-layers, and 

 their first important differentiations into neural tube, chorda, and 

 primitive segments, as well as the origin of the blood and connective 

 tissues, it will be our next undertaking to make ourselves acquainted 

 with the development of the external form of the body, and with the 

 development of the embryonic membranes, the latter being intimately 

 connected with the former. 



There exists an extraordinary difference in these respects between the 

 lower and higher Vertebrates. When the embryo of an Amphioxus 

 has passed through the first processes of development, it elongates, 

 becomes pointed at both ends, and already possesses in the main 

 the worm-like or fish-like form of the adult animal. But the higher 

 we ascend in the series of Vertebrates, the more are the embryos, 

 when they attain the stage of development corresponding to the 

 Amphioxus embryo, unlike the adult animals: at this stage they 

 assume very singular and strange forms, inasmuch as they become 

 surrounded by peculiar envelopes and are provided with various 

 appendages, which subsequently disappear. 



