THE ANCESTRY OF THE CHORDATA. 547 
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 of 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 
Dendroceles, Chztopods, 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 
