310 



EMBEYOLOGY. 



and the characteristic form of the embryo to be more particu- 

 larly declared. We shall speak of the different appearances in 

 groups, as they are associated with the several laminae of the 

 germinal membrane, tracing each principal formation, and 

 each individual organ, in its progress from the beginning to 

 the end of the period we are now considering. 



[ 488. The dorsal laminae have increased in size, and the 

 rudiments of the vertebrae within them (the vertebral laminae) 

 are growing both anteriorly and posteriorly (fig. 339, h, h) ; 

 they surround the spinal canal on the sides, are also to be seen 

 over the medulla oblongata, and several even exist anterior to 

 the ear (fig. 340, at d). In the vicinity of the chorda dorsalis, 



outwardly, between it and the 

 vertebral laminae, arise the 

 first cartilaginous rudiments of 

 the bodies of the vertebrae, 

 which blend superiorly with 

 the laminae of the vertebral 

 arches, close in the canal of 

 the spinal cord below, and 

 surround the cartilaginous co- 

 lumn (sheath) of the chorda 

 dorsalis. Towards the fifth 

 day the chorda dorsalis begins 

 to disappear ; the spinal cord 

 is laterally compressed, and 

 falls into two halves, each of 

 which is again divided into 

 an upper and an under fas- 

 ciculus. It is on the fifth day 

 that the rudimentary enlarge- 

 ments or processes, indicative 

 of the position of the future ex- 

 tremities, make their appear- 



Fig. 340. Anterior end of an 

 embryo somewhat more highly mag- 

 nified, and a few hours older than 

 that of fig. 339. a, a, Cranial in- 

 volucrum ; b, b, vertebral laminae 

 near the crests of the now closed 

 dorsal laminae; c, spinal cord pass- 

 ing into the medulla oblongata, d, 

 which in its turn passes by a de- 

 pression (the fourth ventricle) into 

 the corpora quadrigemina, e ; f, 

 mesocephalon (thai ami and crura 

 cerebri) ; g, hemispheres ; n, supe- 

 rior maxillary bone ; t, auditory 

 vesicle ; k, branchial arches ; I, 

 atrium cordis ; m, the heart hang- 

 ing forwards ; n, bulb of the aorta. 



ance ; the earliest traces of the 

 cerebral envelopes were already 

 conspicuous on the fourth day. 

 The medulla oblongata (fig. 

 340, between c and cl) is ex- 

 tremely flat above, in conse- 

 quence of the divergence of the superior fasciculi from one 



