182 H. V. WILSON AND BLACKWELL MARKHAM 



Lereboullet records some direct evidence for the teleosts. In 

 case no. 56 (loc. cit., p. 225, figs. 32, 33, and 34) he watched the 

 actual development of the embryo from day to day. Anteriorly 

 and posteriorly some fusion of the two half-bodies apparently 

 took place, although as late as the seventh day the half-bodies 

 were still separate throughout an extensive part of the trunk, and 

 no further fusion took place during the remaining five days of 

 the embryo's life. It is, however, probable that the now common 

 view, which Lereboullet advanced, applies to this case and that 

 under favorable circumstances the two half-bodies would have 

 completely fused. The truth of this idea is made almost certain 

 by cases like LerebouUet's no. 57 (loc. cit., p. 226), in which the 

 half-bodies have been brought so close together as to be practi- 

 cally in contact, and his no. 59 (loc. cit., fig. 35), in which at 

 the posterior end of the single trunk there is found a small 

 dorsally situated aperture, interpreted as the remnant of the 

 blastopore. 



O. Hertwig, in his well-known paper ('92) on frog embryos 

 exhibiting the spina-bifida defect, describes many such embryos 

 in anatomical detail. He arranges them in a series which he 

 interprets as representing the actual ontogeny. His series be- 

 gins with the so-called 'ring embryos,' in which the exposed yolk 

 area is very large and the part of the axial body lying in front of 

 the blastopore lip very small — so small, indeed, that it includes 

 no more or scarcely more than the anterior cerebral commissure. 

 In such embryos he concludes with Roux ('88) that the lateral 

 lips of the blastopore become organized into half-bodies, which 

 gradually meet in the midline. And the individual frog embryos 

 which he describes and arranges in a series represent, he thinks, 

 successive stages in the actual ontogeny of such a 'ring embryo.' 

 It is far from certain that all of the abnormalities, so arranged, 

 really belong in a single ontogenetic series. Nevertheless, the 

 data, recorded with admirable precision, doubtless justify the 

 conclusion that sometimes the lateral lips do organize and come 

 together in frogs, as Lereboullet had already claimed was the 

 case in fishes. But Hertwig has no observations on the actual 

 occurrence of this process in one and the same embryo. 



