EMBRYOLOGY OF CLEPSINE. 293 



Thus the order of axial differentiation is in harmony with the 

 supposed phylogenetic order of development. A point of con- 

 siderable theoretical interest demands attention in this 

 connection. In Clepsine, as in numerous other forms^ a well- 

 marked bilateral arrangement appears before the definite gastrula. 

 This is only one of the many instances of what Hackel (4't\) 

 has called ontogenetic acceleration (heterochrony), and it finds 

 an explanation in the principle of precocious segregation (Lan- 

 kester ('-fi) )• Every definite differentiation of material 

 presupposes a preliminary segregation. Clepsine furnishes some 

 striking illustrations of this fact. Nerve-cells are preceded by 

 neuroblasts, these by a primary neuroblast, and the latter by 

 antecedent stages of segregation. The same is true of the 

 mesodermic and entodermic elements. Precisely when this 

 segregation begins it is impossible to say ; but it is certain 

 that it begins in the great majority of cases long before cleavage. 

 So likewise every ontogenetic form presupposes a preliminary 

 arrangement. A radiate arrangement precedes the gastrula ; a 

 bilateral arrangement the definitive bilateral form. Thus it 

 happens that, before a given ontogenetic stage is completed, 

 the preliminary segregations and arrangements for the following 

 stage are already more or less advanced. Thus the gastrula 

 — and more rarely the blastula — is pre- stamped with the 

 antimeric character of the ultimate bilateral form. Such ante- 

 cedent segregations and arrangements illustrate the tendency to 

 concentration in ontogenetic recapitulation. This concentration 

 does not, however, essentially disturb the pahngenetic order of 

 events. 



Let A, B, and C represent three successive phylogenetic 

 forms ; thus : 



A I B j C 



The ontogenetic concentration would then be represented by the 

 same forms progressing side by side ; thus : 



A 



B 



C 



If only the extremities of the lines are kept in view, it will be 

 seen that the palingenetic order is preserved. 



I. Cleavage-cavity (Blastocel, Huxley.) 



A morula, as defined by Hackel (/vV)? does not occur in the 

 ontogeny of Clepsine. Before subscribing to tTie" opinion that 

 such a stage really belongs to the ontogenetic series, some more 

 convincing proof than mere surface views must be adduced in 

 its favour. In most cases where cleavage has been subjected to 



