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III. 

 DEVELOPMENT OF THE F<ETU8. 



> preceding chapters, wo have seen how tho fecundated oviiluin 

 1 to furnish the earliest lineaments of tho foetus and tho organs 

 vd to it ; this chapter will be devoted to an examination of tho manner 

 in uliirh the fn.'tus is developed. 



\mig creature is designated an embryo during tho early period of 

 gestu- ' it has assumed any definite shape ; but as soon as it exhibits 



the f >nn of tho species to which it belongs, it is named n /<rtu*. Tho 

 transition between these two states, though they have different nani< 

 altogether inappreciable ; so that wo content ourselves with studying the 

 first phases of development under the heading of tho formation of the 

 yo, and the last under that of tho development of tho organs. 



ARTICLE I. FORMATION OF THE EMBRYO. 



When the embryo has assumed tho form of an elongated streak, and 



- in its middle the primitive groove, there appear in tho middle lamina 



of tin- blastoderm the chorda dorsalis, tho vertebral lamina:, and the lateral 



DEVELOPMENT OF THE CHORDA DORS A LIB AND VERTEBRAL LAMINA. 



The chorda dortalis is a cylindrical cord, slightly attenuated at 

 both extremities, which is developed beneath tho primitive groove. On 

 its sides is a series of small opaque quadrangular masses the vertebral 

 lamina which are nothing more than tho protuverii l<r<i, <>r first traces of 

 the foetal vertebra. Each of these little masses is soon perforated by an 

 opening, and is then divisible into three portions: the protovertebral caritij, 

 the muscular lamina, situated above the cavity, and the protocertebrie, placed 

 below it. 



muscular lamiiur, in augmenting in volume, are inflected upwards, 

 and unite in tint middle line of tho back. They form, in greut part, tho 

 muscles of the vertebral furrows; they also give off, below, prolongations 

 which concur in the devi -lnpim -nt of the int -rrustal and abdominal muscles, 

 as well as those of the extremities. 



l>rotovcrtebroe curve upwards and downwards in such a fashion as to 



surround the i bral cavity or medullary canal, and the dorsal cord. 



:ior rintf is the rudiment of tho annular portion of the vertebra, 



and tho inferior ring and dorsal cord constitute tho vertebral bodies and 



the discs uniting them. 



DEVELOPMENT OF THE LATERAL AND CEPHALIC LAMINA. 



The lateral /.I//M'//.I is tho name given to that part of the middle laminu 

 of tho blastoderm wliich is plan d to the right und left of tho vertebral lainime. 

 In the region of the trunk, these lamiiue are separated for a certain \ 



