466 T. H. MORGAN. 



principles. Although many naturalists still admit the prin- 

 ciple of use and disuse as a factor of organic evolution, perhaps 

 an equally large number reject the explanation. Hence, until 

 we get definite proof of the truth or falsity of any such tlieory, 

 it ought not to be used as the starting-point on which to buikl 

 up other or new theories. 



Caldwell (11) has offered a brief and interesting attempt to 

 explain metameric repetition. So brief, indeed, is the presenta- 

 tion^ and so obscurely worded, that I am not certain that I 

 have entirely understood the meaning of the author. It is a 

 mechanical theory par excellence. The theory assumes 

 that as early as the blastula^ stage the endoderm, as well as 

 the mesoderm, is represented in cells or groups of cells at the 

 surface of the sphere. Now the endoderm may before inva- 

 gination get separated into two parts, owing to an early 

 elongation of the blastula, so that when gastrulation sets in it 

 may take place at two regions of the surface. One of the 

 regions may contain much more endoderm than another, 

 giving an oral or an anal gastrulation as a result. Similarly 

 the mesodermal ^' Anlage " may be pulled apart and turn in with 

 the endoderm at one region or the other, or may even have 

 been left along the line where the two endodermal masses 

 separated. 



In many cases the whole of the mesoderm may be turned 

 into the gastrula cavity with the endoderm, and subsequently 

 set itself free from the endoderm by one, two, or many gut- 

 pouches. When many gut-pouches arise, they mark the be- 

 ginning of metameric repetition. The reason for many pouches 

 appearing in some forms is to be explained as due to an early 

 elongation of the invaginated endoderm forming the archen- 

 teron, so that the mesodermal "Anlagen^' get pulled out and 

 broken apart. 



If, as Caldwell supposed, there is pre-formatiou for the 

 mesoderm, there must be for all the other organs of the body, 

 and this the author admitted. 



' The author says planula, which makes his explanation obscure ; unless 

 he means technically a blastula. 



