328 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



distinguish cases in which the separation of the future individuals 

 occurs during cleavage from those in which the process concerns 

 later stages. In the former cases, the separate individuals are 

 isolated by the process of cleavage itself, whereas in the latter, 

 processes of dichotomous growth and fission are involved. 



In the former category, we first have certain cases in which 

 repeated and irregular division, leading to separation, occurs at an 

 early stage of cleavage. This phenomenon, usually called poly- 

 embryony, is found in certain Hymenoptera and Polyzoa. Here, 

 this process leads to the production of numerous separate in- 

 dividuals from one egg by the separation of its blastomeres or 

 groups of blastomeres. These cases are really natural experiments 

 of blastomere isolation, and it may be noted that axes of polarity 

 and symmetry relations play little part in the process. As to why 

 it is in these cases that the blastomeres separate and produce 

 wholes on their own instead of parts, little can be said except to 

 point out that in Hymenoptera and Polyzoa the fertilised egg 

 undergoes cleavage within a mass of living matter, consisting in 

 the case of the former of the tissues of a parasitised caterpillar 

 preyed upon, and in the case of the latter, of the nutritive cells of 

 the ovicell or brood pouch. ^ In these cases it is interesting to note 

 that a fertilised frog's egg grafted into the body cavity of a fully 

 developed frog undergoes modified cleavage and these products 

 become separated and develop as far as they are able on their own.^ 



Another group of cases comprises those where the early cleavage 

 stages are artificially interfered with in one way or another. This 

 phenomenon leads to the formation of double (or multiple) monsters, 

 each partner being derived from a blastomere or group of blasto- 

 meres which has been to a certain extent isolated, physiologically 

 or physically, from the others. Here we must place the double 

 monsters obtained in Amphioxus as a result of shaking and dis- 

 arranging the blastomeres (pp. 79, 123): in Tubifex and in Chae- 

 topterus as a result of inducing equal divisions of blastomere D 

 containing the essential ingredients for the formation of somatoblasts 

 (twinning in Clepsine is probably of this type (p. 113)) : in the star- 

 fish Patiria as a result of spontaneous parthenogenetic development 

 resulting in semi-independent development of both of the blasto- 



1 Harmer, 1930. ^ Belogolowy, 1918. 



