1032 



EMBRYOLOGY. 



The following are the weights of the completely developed foetuses. 

 The iveight of the foetus of the Mare, at birth, varies according to breed. It 

 has been found to be 35 pounds in a Corsican Mare, and 135 pounds in a Suflfolk- 

 Boulonnaise Mare ; and between these extremes are many intermediate weights. 

 In Cattle, the weight of the Calf is, on an average, 2*31 parts that of the Cow. 

 According to Leuckart, the weight of the Puppy at birth is about one 

 pound ; that of the Lamb, ten or eleven pounds ; 

 that of the Pig, five pounds ; that of the Chick, 

 about one and a half ounce. 



The transition between the embryonic and the 

 foetal condition being inappreciable, we will study 

 the first phases of development under the heading 

 of formation of the embryo, and the last under 

 that of the development of organs. 



Article I. — Formation of the Embryo. 



When the embryo has assumed the form of an 

 elongated streak, and shows in its centre the 

 primitive groove, there appear in the middle 

 lamina of the blastoderm the chorda dorsalis, the 

 vertebral lamince, and the lateral lamince,. 



Development of the Chorda Dorsalis and 

 Vertebral Laminae. 



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 /«mw««— which are nothing 

 more than the jjrotovertebrce, or first traces of the 

 foetal vertebrae. They appear in the embryo, on 

 each side of the neural axis, in the form of dark 

 quadrilateral spots, and are developed in a series 

 in the spinal zone. 



They are hollowed by a central cavity, to- 

 wards which converge a radial series of cells. On 

 their lateral surfaces they are continuous with the 

 musculo-cutaneous and fibro-intestinal lamina, by 

 means of an intermediate part — the intermediate cellular lamina. 



At the third day they give rise, by differentiation, to the muscular lamincB 

 (Fig. 567, m.s.). The protovertebrse afterwards become fused, and invest all the 

 neural tube and the chorda dorsalis ; and it is in this homogeneous matrix that 

 an ulterior segmentation marks the traces of the definitive vertebrae. The articular 

 intervals in the latter correspond to the middle portion of the proto-vertebrae. 



The muscidar lamince, 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. 



EMBRYO OF THE CHICK AGED 

 ABOUT THIRTY-SIX HOURS (IN- 

 FERIOR face). 



FB, Anterior brain, on the sides of 

 which are seen the optic vesicles. 

 op; s.o, posterior limit of the fold 

 of somatopleure ; HB, posterior 

 brain ; MB, middle brain ; p.v. 

 and v.pl, protovertebrae ; ch, an- 

 terior extremity of the notochord ; 

 pi, outline of the area pellucida; 

 pv, primitive trace. 



