THE ANCBSTEY OF THE CHORDATA. 79 



occurrence. To such an extent is this true that in a recent 

 contribution to this subject (Caldwell, 'Quart. Journ. Mic. 

 Sci./ 1885), a suggestion has been made which proposes to 

 give a simple physical explanation ot all the phenomena of 

 segmentation. Caldwell suggests that owing to the early 

 acquisition of the long axis of the body and the consequent 

 elongation of the blastopore, the mesoblast has become, so to 

 speak, left behind in blocks, in consequence of the more rapid 

 growth of the epiblast. That this extremely simple theory 

 will not account for all cases of repetition is shown, firstly, by 

 the fact that though the repeated structures are generally me- 

 soblastic, yet they are not always so ; secondly, that the meso- 

 blast does not thus originally segment as a whole, but rather 

 that separate organs repeat themselves separately, as has been 

 already urged, especially in the case of the Turbellaria; and 

 finally, these repetitions are by no means universally embryonic 

 or even larval features, but their whole history rather points to 

 their having very generally originated in the adult condition, 

 and to the view that they have come to be thus earlier in 

 development, the opposite of which is assumed by such a 

 hypothesis as Caldwell's. 



This belief that these repetitions have had their origin in 

 variations which occurred in the first instance late in life is 

 founded upon several considerations. Firstly, the cases in 

 which the generative organs are repeated are very numerous; 

 in fact, both organs or the testis, at all events, are repeated in 

 nearly all the cases in which much repetition is found (in most 

 Dendrocoeles, Chaetopods, Nemertines, Balanoglossus, Am- 

 phioxus), even if few other systems are repeated. In the 

 case of these organs it is most likely that the repetition first 

 arose in adult life, and, in fact, in most of them it does still 

 so arise ; that is to say, the masses of cells which are to form 

 generative organs are not specially broken up at an early age. 

 And in the second place, the original late origin of repetitions 

 is likely from the fact that most of them still so arise ; it is 

 only in exceptional cases as that of the mesoblastic pouches of 

 Vertebrata, Phoronis, Enteropneusta, and the horns of the 



