author's abstract of this paper issued 

 by the bibliographic service, january 19 



ASYMMETRICAL REGULATION IN ANURAN EMBRYOS 

 WITH SPINA BIFIDA DEFECT 



H. V. WILSON AND BLACKWELL MARKHAM 



FIVE FIGURES 



In fishes and amphibia it frequently happens that something 

 interferes with the normal movement of the blastopore lip over 

 the yolk. In these cases, familiar to embryolo gists, the anterior 

 end of the axial body develops in front of the blastopore lip and 

 is continuous behind with the two halves of the latter. Between 

 these lateral halves in amphibia there lies bare yolk, whereas in 

 fishes the exposed yolk mass may become completely or nearly 

 covered in (Lereboullet's figures, '63) by a thin layer of tissue 

 produced as an extension from the blastopore lips toward the 

 median line. The embryos in the two groups are, however, 

 essentially alike. 



A survey of the facts leads to the following conclusions as to 

 the ways in which such abnormalities may continue to develop: 



1. Overgrowth may be merely slowed up, in which case noth- 

 ing strikingly abnormal results. 



2. The lateral halves of the blastopore lip organize into 'half- 

 bodies.' Each half-body becomes a whole, and thus a double 

 monster results. Hertwig ('92, p. 464) denies the possibility of 

 this occurrence. But it was established as a fact by the obser- 

 vations recorded in Lereboullet's classic paper ('63; cf. especially 

 Lereboullet's figs. 26 and 27, pi. Ill, and figs. 28 and 29, pi. III). 

 Roux maintained it ('88, p. 516) and Endres further established 

 it ('96, p. 541). Lereboullet actually observed (loc. cit. passim; 

 cf. pi. 2, fig. 6) in a number of cases the gradual fusion of the two 

 bodies of a double embryo (formed perhaps in other ways than 

 the above) into one body. This, then, would be one way open 



171 



