HEGULATiON IN ANURAN EMBHYOS 177 



have only been touched upon in a preliminary, reeonnoiterinp; 

 sort of way. BrilHant as the results are, they need to be inter- 

 woven with rich collections of special knowledge, which shall 

 combine the data of experimental organogeny with tabulations 

 of the various organogenetic methods practiced by sets of indi- 

 viduals, and races, of a species and by related species. Twenty- 

 five years ago, one of the writers mapped out in a tlescriptive 

 paper (Wilson, '94) this plan as affecting the study of the com- 

 parative embryology of sponges. The idea was to learn some- 

 thing, through comparison, of the kinds of changes that may be 

 made in the ontogenetic processes of a race and the laws govern- 

 ing their appearance. The comparative study remains as im- 

 portant to-day as it was then, and the field is almost as virgin. 

 In a word, before accepting this conclusion that the end sets in 

 activity the means, we need to know in any particular case much 

 more both about it and allied cases. Ontogeny we picture as an 

 intricate series of events, each event setting free stimuli which 

 bring about other events. If, then, a certain event does not 

 occur, the embryo in respect to this point may, for all we know, 

 fall back to a lower level, to a more generalized ancestral condi- 

 tion, in which it makes responses such as it would have made in 

 former times. Or it is possible that steps in ontogeny not directly 

 dependent on the occurrence of the inhibited event may continue 

 to be made, one after another, and thus we may come out with 

 an embryo in which only a part is missing, in which case the 

 missing part might conceivably be supplied as if it had developed, 

 had been lost, and now were being regenerated. All this, of 

 course, is conjecture, but it is at least physiological. 



Liver. The liver appears as a large cellular mass excavated 

 by abundant sinuses which divide it up into cords. It is inti- 

 mately connected, throughout most of its extent, with the yolk 

 mass, shading off into the latter in places. 



Spinal cord. The spinal cord in the trunk region, as far back 

 as the base of the right tail, is symmetrical, one side having the 

 same amount and arrangement of incipient gray and white mat- 

 ter as the other. At the base of the right tail, the spinal cord is 

 continued into a tube with a much wider lumen. The wall of 



