538 PATTERNS AND PROBLEMS OF DEVELOPMENT 



tertiary; etc/' The cell masses may differ considerably in size, and, accord- 

 ing to Patterson (192 1), an embryo may develop from a single cell. On 

 the other hand, many masses fail to develop in some species, and some give 

 rise to asexual, nonviable individuals. There seems to be no common ori- 

 entation of the axes of embryos in the polygerm, nor do all embryos de- 

 velop at the same time or from cell masses of the same generation. There 

 is no evidence of persistence of axiate pattern through all the divisions of 

 the polygerm, and the apparently fortuitous character of the divisions sug- 

 gests that, if pattern was originally present, it has been obliterated. Can 

 axiate pattern originate autonomously in the final generations of cell 

 masses? If a gradient pattern, a molecular pattern, or a spatial pattern of 

 any kind determining axiate organization of the individual insect does 

 arise de novo, it is difficult to conceive how this is possible, genetically or 

 otherwise, except in relation to some initiating factor external to the mass 

 concerned. Differentials in oxygen supply or in CO2 accumulation and 

 perhaps potential differences in the parent body and between the masses 

 may be factors in initiating pattern. Some of the figures given in papers 

 cited above suggest that the final polarities of embryos may be determined 

 by a differential in their local environment — for example, between the 

 wall and the interior of the polygerm — but the question is not discussed by 

 the authors. Failure of some masses to develop may be due to absence or 

 inadequacy of axiate pattern rather than to nutritive conditions, as sug- 

 gested. Development of masses of any generation and failure of many to 

 develop suggests that growth and division of the masses continues until a 

 mass acquires a pattern adequate for development. In vitro cultivation of 

 the polygerms or cell masses, if found to be possible, may give some infor- 

 mation concerning their developmental physiology. 



In most vertebrates polyembryony is only occasional under natural 

 conditions and is limited to various degrees of twinning, ranging from all 

 degrees of teratological duplication to conjoined twins, equal or unequal 

 in development, and completely separate duplicate or identical twins. 

 The descriptive Hterature of vertebrate teratology is voluminous, and the 

 question whether duplications and multiple forms result from fusion of 

 originally separate embryonic primordia or from division of a single pri- 

 mordium has been discussed for many years. However, continued study 

 of the teratological forms and of complete twins and the experimental 

 embryonic reconstitutions have estabhshed beyond question the origin 



"Marchal, 1904; Silvestri, 1905, 1906, 1908; Patterson, 1915, 1921; Gatenby, 1918; 

 Leiby, 1922; and literature cited by these authors. 



