REGULATION IN ANURAN EMBRYOS 187 



ranted. He recognizes that the fish body is first marked out as 

 a projection reaching forward from the blastodermic rim. This 

 'tubercle,' in normal development as in double-embryo formation 

 where two tubercles are formed, elongates to form a linear body 

 ('bandelette embryonnaire'). Lereboullet speaks of the pro- 

 duction of the original tubercle as due to a sort of 'vegetation' 

 from the blastodermic rim (a description that is clearly not war- 

 ranted). As to the processes involved in its elongation, he is 

 not explicit. But it would seem that he means that the tubercle 

 elongates by its own growth, possibly incorporating the neigh- 

 boring tissue of the blastodermic rim (which no one would deny) . 

 By contrast, in his fourth kind of anomaly, what we call to-day 

 the spina-bifida embryo, the original tubercle does not grow and 

 elongate, but the lateral blastodermic edges organize and par- 

 ticipate directly, as such, in forming the body. In fact, in his 

 search for the growth processes that lead to his various anom- 

 alies, he gives the impression of having distinctly in mind the 

 power of the embryo to proceed to the type structure in more 

 ways than one. ^Tiereas, some of the later embryologists, like 

 O. Hertwig, assume for the moment at least that ontogeny always 

 exhibits the same series of events, anomalies being due to retard- 

 ation or acceleration in the occurrence of particular events. 

 While undoubtedly many anomalies are produced in this way, 

 there are as certainly others the causation of which is much more 

 subtle and complex. 



SUMMARY 



Bufo and Chorophilus embryos occur in which, when blastopore 

 closure is inhibited, an asymmetrical regulatory process comes 

 into activity. Instead of the two lateral blastopore lips fusing 

 in the midline, the blastopore is shifted over toward one side, and 

 from a single lip a backward extension of the axial organs is pro- 

 duced. Such a tadpole was reared to a stage in which external 

 gills had been absorbed, internal gills and opercular cavity formed. 



University of North Carolina 

 Chapel Hill, North Carolina 



