CHAP. X.] THE DEVELOPMENT OF THE CAT. 335 



The manubrium ossifies after the other segments of the sternum, 

 which seem each to ossify from one centre. 



§ 9. The DEVELOPMENT OF THE SKULL takos place in a specially 

 circuitous manner, so that its early stages are strikingly unlike its 

 mature condition. The first indication of the future skull is given 

 hy the expansion, before mentioned, of the anterior end of the 

 medullary groove, which expansion, as has been said, becomes 

 divided by two lateral constrictions (one in front of the other, on 

 each side) so that three rounded vesicles are formed lying serially 

 one before the other. The notochord extends forwards to beneath 

 the second of these vesicles, which bend down sharj^ly in front of 

 its anterior termination, so that there comes to be one vesicle 

 above, one in front of, and one below the anterior termination of 

 the chorda. 



These vesicles are, as we shall hereafter see, the commencements 

 of the futm'e brain. 



In the walls of the ascending lamina? dorsales, which bound 

 the vesicles laterally, there are no quadrate thickenings like those 

 developed on each side of the chorda in the vertebral region, 

 while peculiar developments take place in their ventral lamina3. 

 For while the medullary groove is being arched over and con- 

 verted into the great axial, neural canal, by the ascending lamina) 

 dorsales, another axial canal is being formed beneath the neural 

 one bv the desccndin"; lamina ventrales. This second axial canal 

 is the rudimentary alimentary one. The ventral laminse, as they 

 bend down to enclose the incipient pharynx, grow thinner and 

 thinner at successive intervals, one behind the other, till a series 

 of perforations, the risceral clefts, are formed, one after the other, 

 each cleft leading from the exterior into the pharyngeal cavity. 

 Four such visceral clefts appear on each side. The perforation 

 proceeds from within outwards, the hypoblast being absorbed first, 

 then the inner part of the mesoblast, and finally, the whole of the 

 mesoblast, the hypoblast growing outwards along each advancing 

 wall of each aperture, and ultimately becoming continuous with 

 the epiblast. In front of each cleft the wall of the ventral lamina 

 becomes more or less thickened, forming what are called the 

 visceral arches (Fig. 152, ^, ^, ^, ^) — each such lateral series of arches 

 being at first separate from their fellows of the opposite side, as 

 are the ventral laminse themselves. Meantime another pair of 

 vesicles — the cerebral vesicles — grow forwards (side by side) from 

 that which was at first the most anterior vesicle ; and a pit formed 

 beneath the outer anterior part of each cerebral vesicle lays the 

 foundation of the future nasal organs (Fig. 152, na), while two 

 other and more posterior invaginations on each side (one beneath 

 what was the first vesicle and the other beside the hindmost) 

 respectively lay the foundations of the eye and of the ear. The 

 mouth is formed by a superficial depression — as will be subsequently 

 more fully explained. On each side of the mouth the first visceral 

 arch has meantime grown down and united distally with its fellow of 



