REGULATION IN ANURAN EMBRYOS 183 



In fact, direct observations on the occurrence of the symmet- 

 rical fusion of half-bodies are, as we have said, few and scanty. 

 They lack detail. Nevertheless, Roux ('88, p. 443) has actually 

 watched the gradual approximation of the lateral lips of the blas- 

 topore in frog embryos exhibiting the spina-bifida defect (desig- 

 nated by him, consistently with his theory, asyntaxia medul- 

 laris), until the blastopore had quite closed. 



This symmetrical mode of development of spina-bifida em- 

 bryos is looked on by many embryologists (cf. especially Roux, 

 '88, pp. 443-444, and O. Hertwig, loc. cit.) as not fundamentally 

 different from the normal embryology. The difference, accord- 

 ing to these writers, is that, whereas in normal development the 

 lateral blastopore lips come together before they are organized, 

 in the spina-bifida embryos their fusion is so delayed that they 

 become organized first. Thus the organization of the blastopore 

 lips in a spina-bifida embryo is not looked on as an abnormal reg- 

 ulation that starts up when something checks the activity of the 

 morphogenetic process by which the body is usually lengthened. 

 It is, on the contrary, a process that is perfectly normal in itself, 

 only out of place in time. This is the point of view of the con- 

 crescence theory of vertebrate ontogeny. 



Undoubtedly, symmetrical spina-bifida embryos strongly sug- 

 gest the idea of concrescence — so strongly, indeed, that they 

 are thought of by some as almost demonstrating the truth of its 

 occurrence in normal development. But this reasoning is ob- 

 viously vicious, since it begs the question as to whether organi- 

 zation of the lateral lip is a normal process (although out of place 

 in time) or a radically abnormal one. Such radically abnormal 

 regulatory processes of course occur. Driesch and many others 

 have made us far more familiar with them than were the older 

 embryologists. And in establishing or restoring the type struc- 

 ture, their disregard of the customary mode of attaining that end 

 is well known. Following this line of thought, we may entertain 

 the idea that the organization of the blastopore lip in the embryos 

 in question is a thoroughly abnormal process, as far away, al- 

 though in a different direction, from the normal mode of forming 

 the axial body as is the process, for instance, in those rare teleost 



