494 , EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



frequently traversed by a fine superficial groove which passes 

 forwards along the median line. 



II. Then there occur curious cases of abnormality in which the 

 dorsal region of the body is actually divided into two halves by a 

 longitudinal split in the mesial plane. Thus Oscar Hertwig found 



Fi'.. 220. Abnormal t-mliryos illustrating the Protostoma theory. 



A, abnormal Prog aoibryo Men from the dona] sidr -, is. transverse sn-tiiiii through hinder third of 



iliLt-. (att.-i <). Hi-rtwin, I sir.'); ( '. aluicinnal Trout rinbryo (>'. jurio) in dorsal view (alter K<>|.M-|. 

 I), al>ni>rnial embryo ol I'iki- (/>' I in in-) (.-il'in I ,n fUuillrt . ISi'.ii). m.f. niedullai v fold : m.a, nirsndciiii 

 iii-s.Kl.'nii ; N, nolocliiinl : '/. opi-iiin- li'.'idin- down into micron ; ot, otocv ^t ; i/. mass 

 ot yolk-. -i-ll-. 



(1892 ; l.li.-it 1'V i'crtili/iiiM frog's r^s which liad 'becninc 

 .-iihi-r liy iviciiiinii within tin- oviduct or by li'in# kept for from one 

 tofourdiiv- in ;i iin.isl chainht-r. In- <>l>lainril a ccrlniii niinil' 

 ahiioruiid Mnlrys f Ihc i\]c shown in Ki<^. 220, A, where a lar^o 

 086 "1 yolk-Cellfl is visillr in dorsal \ir\s instead <>!' hcin^ rt>m- 

 plcl.-ly OOVered in ae would In- (,lie case normally. lii IranswTs.' 

 s?<;tio!i CFi^. 220, H; such an c.mbryu \\.is found l lia.\- I\MI halt' 



