BELL-GASTRULA. 199 



Although the animals which we have named belong to 

 the most diverse classes, they all have this in common 

 with each other and with many other animals, that, owing 

 to constant heredity, they have retained the palingenetic 

 form of egg-cleavage and Gastrula-formation, which they 

 received from their oldest common ancestors, up to the pre- 

 sent day. This is, however, not true of the large majority 

 of animals. On the contrary, in them the original process 

 of germination has, in the course of many million years, 

 gradually changed in a greater or less degree, and has 

 become vitiated owing to adaptation to new conditions of 

 evolution. , Both the egg-cleavage, or segmentation, and 

 the formation of the Gastrula, or gastrulation, which 

 succeeds the segmentation, have in consequence of this 

 acquired an aspect which is in many ways different. In 

 the course of time the differences have even become so 

 marked that the cleavage process of most animals was 

 wrongly interpreted, and the Gastrula of these animals was 

 altogether unknown. It is only owing to the extensive 

 comparative researches which I instituted in late years 

 among animals of the most diverse classes, that I have been 

 enabled to indicate the one common process which under- 

 lies those processes of germination, apparently so different, 

 and have traced back all the diverse forms of germination 

 to the one original form, the form which has already been 

 described. To distinguish them from this primary palin- 

 genetic form of germination, I shall call all the secondary 

 forms, varying from the primary, vitiated, or kenogenetic 

 processes. The more or less varying Gastrula-form, which 

 results from this kenogenetic egg-cleavage, may be called, 

 generally, the secondary, modified Gastrula, or Metagastrula. 



