CHAPTER III. 



DEVELOPMENT OP THE FCETUS. 



In the two preceding chapters, we have seen how the fecundated ovulum 

 is modified to furnish the earliest lineaments of the foetus and the organs 

 annexed to it ; this chapter will be devoted to an examination of the manner 

 in which the foetus is developed. 



The yoimg creature is designated an embryo dvLTing the early period of 

 gestation, before it has assumed any definite shape ; but as soon as it exhibits 

 the form of the species to which it belongs, it is named a fcetus. The 

 transition between these two states, though they have different names, is 

 altogether inappreciable ; so that we content ourselves with studying the 

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

 embryo, and the last under that of the development of the organs. 



Article I. — ^Foemamon op the Embeto. 



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

 shows in its middle the primitive groove, there appear in the middle lamina 

 of the blastoderm the chorda dorsalis, the vertebral lamince, and the lateral 

 laminae. 



DEVELOPMENT OF THE CHOEDA DOESALIS AND VEETEBEAL LAMINiE. 



The chorda dorsalis is a cylindrical cord, slightly attenuated at 

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

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

 lamince — which are nothing more than the protovertebrce, or first traces of 

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

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

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



below it. . a -1 J 



The muscular laminae, in augmenting in volume, are inflected upwards, 

 and unite in the middle line of the back. They form, in great part, the 

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

 which concur in the development of the intercostal and abdominal muscles, 

 as well as those of the extremities. _ t, • i 



The protovertebrce curve upwards and downwards in such a fashion as to 

 surround the protovertebral cavity or medullary canal, and the dorsal cord. 

 The superior ring is the rudiment of the annular portion of the vertebrae, 

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

 the discs uniting them. 



DEVELOPMENT OF THE LATEEAL AND CEPHALIC LAMIN*. 



The lateral lamince is the name given to that part of the middle lamina 

 of the blastoderm which is placed to the right and left of the vertebral laminae. 

 In the region of the trunk, these laminae are separated for a certain period 



