ON THE riASTTlULATTON TN PETROMYZON. 39 



Fiif. 7iy — Side view of a further advanced ovum. The constriction of the 

 boundary groove (A.j/.) is not only strikin«.^, but now surrounds 

 the ovum completely. While the invagination is further 

 carried on, the basal surface is depressed and flattened further 

 ventrally. 



Fi'c/. IJ) — üorsal view of the same ovum. The embryonic shield (em.s.) 

 has become broader, and the ))Oundary groove (h.g.) has shifted 

 anteriorly in consequence of the advancing invagination. 



Fig. Ic — lîasal view of the same ovum. The invaginating groove {hp.) is 

 nuich extended, describing an almost semicircular arc. It is 

 bounded outside by the blastoporic lip {hpJ.) of the same form and 

 which is most prominent in the middle part. The field contained 

 within the arc is depressed a great deal, and the depression is 

 deeper towards the steepest middle part of the blastoporic lip, 

 thus presenting a wide-gaped blastopore of a funnel-like shape. 



Fig. 8a — Side view of an ovum in which the translucent part is reduced, 

 in consequence of the progressing invagination, into a small 

 vesicle at the anterior end of the ovum and is separated from the 

 opaque embryonic shield by the strongly curved boundary groove 

 (&.r/.). 



Fig. Sh — Dorsal view of the same ovum. The larger part of the ovum is 

 occupied by the opaque solid part, the embryonic shield (eon. .<?.) ; 

 capping this is seen the small translucent vesicle at the anterior 

 end. 



Fig. 8c — Basal view of the same ovum. Both limbs of the arc described 

 by the blastoporic lip (bp.i.) are brought nearer to the median 

 line and to each other, while the blastoporic depression has become 

 deeper than before. 



Fig. 9a — Side view of a further advanced ovum. The whole extent of 

 the dorsal surface is occupied by the embryonic shield (e?/i.s.). 

 The translucent vesicle is driven into the ventral side of the 

 anterior part and is seen as a slight swelling. A great deal of 

 the vegetative hemisphere remains still exposed ; accordingly, the 

 ovum is somewht conical in shape, the posterior part being 

 bulged out. 



Fig. dl) — Dorsal view of the same ovum. The translucent vesicle can 

 not be seen, while the boundary groove is perceived as two slight 

 indentations on both sides of the anterior end {h.g.). 



