DEVELOPMENT OF THE SKELETON. 



727 



another has been observed by Mihalkovies in the interval l>etwecn the 

 basi- occipital and the basi-sphenoid bones or their cartila[,nnous matrices. 



Fig. 526. — View from above of the Embryo-Chick in the Fig. 525. 



FIRST HALF OF THE SeCOND DaY. 



1, 2, tbe three primary enceplialic vesicles ; iu front and to the 

 sides the cephalic fold ; crossing at 2 the fovea cardiaca ; 3, tlie 

 caudal extremity of the medullary canal dilated into a rhomboid 

 form ; 4, 4, six primordial vertebral divisions. 



In front of this last swelling, the chorda is beiit 

 down below the base of the sknll, and tapering to a 

 fine filament, ends or is lost in the floor of the 

 pituitary fossa. The enlargements now mentioned 

 have some interest in connection with the question 

 of the vertebral constitution of the sknll 



Segmentation of the Protovertebree. — The 

 transverse vertebrate segmentation which occurs in 

 the primary vertebral plates affects only that part of 

 these plates which is formed of mesoblast. It begins 

 at a very early period, as already stated, even before 

 the close of the primary medullary canal, in the form 

 of one or two, or it may be three short transverse 

 transparent lines which separate a corresponding 

 number of dark or condensed quadrilateral masses of the primitive 

 vertebral plates. These quadrilateral masses, the so-called primordial 

 vertehrcc (Urwirbel of the Germans) (fig. 526, -4, 4), do not, however, 

 correspond merely to the vertebrae of the skeleton, nor are they directly 

 converted into theii- rudiments, but they are rather divisions equivalent 



Fi-. 527 



Fig. 527. — Cervical part of the Piujiitive Verte- 

 bral COLDMX AND ADJACENT PARTS OF AN EmBRYO 



OP THE Sixth Day, showing the division of the 

 Primitive Vertebral Segments (from Kolliker after 

 Remak). 



1, 1, chorda dorsal is iu its sheath, pointed at its 

 nipper end ; 2, points by three lines to the original 

 intervals of the primitive vertebrffi ; 3, in a similar 

 manner indicates the places of new division into perma- 

 nent bodies of vertebrte ; c indicates the body of the 

 first cervical vertebra ; in this and the next the pirimi- 

 tive division has disappeared, as also in the two 

 lowest represented, viz. , d and the one above ; in those 

 intermediate the line of division is shown : 4, points 

 in three places to the vertebral arches ; and 5, similarly 

 to three commencing ganglia of the spinal nerves : the 

 dotted segments outside these i^arts are the muscular 

 plates. 



in number and position to the vertebral seg- 

 ments of the body (somatomes of Goodsir) ; 

 each one comprising superficially a segment of the muscular plate, and 

 more deeply a pair of intervertebral ganglia and nerves, as well as the 

 parts of the skeleton which lie before and behind them. 



The more obvious protovertebral segmentation docs not extend into 



