160 REPRESENTATION OF PROTOMORPHIC ALTERNATION 



tain a proper vitality for some time, but tliey have never 

 been observed to undergo any farther development. It 

 would appear to be different, however, when the general 

 mass divides by a process of fission into two or more equal 

 segments, as is stated by Agassiz to occur occasionally in 

 the ovum of a species of Eolis, for in this case he has ob- 

 served each segment to dev elope a distinct embryo of its 

 own. * 



§ 4. This multiplication of embryos in the ovum of 

 Eolis shows that it is not among monstrosities only that 

 we meet with cases establishing a transition between the 

 phenomena of alternation and those of ordinary embry- 

 ogeny ; and the evolution of the Polyzoa furnishes us with a 

 farther confirmation of the same, for in some of this group 

 we have as a normal occurrence w^hat may be compared to 

 the formation of a double primitive trace, or a double 

 monster, in the development of the vertebrate ovum. In 

 the Polyzoa, as has been already noticed, the immediate 

 product of the ovum is a ciliated germ-mass, like an infu- 

 sorial animalcule, from a protrusion of which a pair of 

 polypes are budded off in succession, the phenomena pre- 

 senting, as Professor AUman observes, '' some remarkable 

 analogies, which would seem to bring the whole process of 

 generation and gemmation in these animals within the do- 

 main of the so-called Law of Alternation of Generations. ""(* 

 Yet the case is not commonly reckoned one of " alterna- 

 tion," and in fact agrees with it only in the multiplication 

 of the secondary products of development, for in the ma- 

 jority of the group, neither the two first formed gemmae. 



* Lectures on Comparative Embryology, quoted by Dr. A. Thomson, 

 in Cyclop, of Anat. Physiol., art. Ovum, p. 2-i. Such a division of the 

 yolk after segmentation into several (foui-) globular masses, I have ob- 

 served myself in the ova of a Nudibranchiate Mollusc, though I have had 

 no opportunity of tracing the development farther. 



t Allman's Polyzoa, p. 41 (Kay Soc.) 



