EMBRYONIC DEVELOPMENT OF PURPURA. 



615 



of theru must include an amount of substance equal to that of a 

 great number of the bodies originally found -within the capsule. 

 The explanation of this fact (long since noticed by Dr. J. E. Gray 

 in regard to Bucci- 



num) seems to be as Fig. 324. 



follows : — Of those 

 500 or 600 Egg-like 

 bodies, only a small 

 part are true Ova, 

 the remainder being 

 merely Yolk - sphe - 

 rules, which are des- 

 tined to serve for the 

 nutrition of the Ein- 

 bryoes. The distinc- 

 tion between them 

 manifests itself at a 

 very early period, 

 even in the first seg- 

 mentation ; for while 

 the Yolk - spherules 

 divide into two 

 equal hemispheres 

 (Fig. 324, b), the real 

 Ova divide into a 

 larger and a smaller 

 segment (d) ; in the cleft between these are seen the minute ' direc- 

 tive vesicles,' which appear to be always double or even triple, 

 although, from being seen 'end-on,' only one may be visible; and 

 near these is generally to be seen a clear space in each segment. 

 The difference is still more strongly marked in the subsequent 

 divisions; for whilst the cleavage of the Yolk -spherules goes-on 

 irregularly, so as to divide each into from 14 to 20 segments, 

 having no definiteness of arrangement (c, e, f, g), that of the 

 Ova takes-place in such a manner as to mark-out the distinction 

 already alluded-to between the Cephalic and the Visceral portions 

 of the mass (h) ; and the evolution of the former into distinct 

 organs very speedily commences. In the first instance, a narrow 

 transparent border is seen around the whole Embryonic mass, which 

 is broader at the Cephalic portion (i) ; next, this border is fringed 

 with short Cilia, and the cephalic extension into two Lobes begins 

 to show itself; and then between the lobes a large Mouth is 

 formed, opening through a short, wide Oesophagus, the interior of 

 which is ciliated, into the Visceral cavity, occupied as yet only by 

 the yolk-particles originally belonging to the ovum (k). 



479. Whilst /these developmental changes are taking place in 

 the Embryo, the whole aggregate of segments formed by the sub- 

 division of the Yolk-spherules coalesces into one mass, as shown 



Early stages of Embryonic Development of 

 Purpura lapillus : — a, egg-like spherule ; 

 b, c, e, f, g, successive stages of segmenta- 

 tion of yolk-spherules ; d, h, i, j, k, successive 

 stages of development of early embryoes. 



