494 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



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 



at ■■■■ 



fns. 



Q 



Fig. 220. — Abnormal embryos illustrating the Protostoma theory. 



A, abnormal Frog embryo seen from the dorsal side ; B, transverse section through hinder third of 

 ditto (after O. Hertwig, 1892) ; C, abnormal Trout embryo (S. fario) in dorsal view (after Kopsch, 1899) ; 

 D, abnormal embryo of Pike (Esox Zucius) (after Lereboullet, 1863). m.f, medullary fold ; m.s, mesoderm 

 segment ; mes, mesoderm ; N, notochord ; 0, opening leading down into enteron ; ot, otocyst ; y, mass 

 of yolk-cells. t 



(1892) that by fertilizing frog's eggs which had become over-ripe, 

 either by retention within the oviduct or by being kept for from one 

 to four days in a moist chamber, he obtained a certain number of 

 abnormal embryos of the type shown in Fig. 220, A, where a large 

 expanse of yolk-cells is visible in dorsal view instead of being com- 

 pletely covered in as would be the case normally. In transverse 

 section (Fig. 220, B) such an embryo was found to have two half 



