CERTAIN GENERAL CHARACTERISTICS 53 



determining its position on the oblique surface. That this is the case is 

 further indicated by occasional development of a head on the more an- 

 terior part of the posterior, as well as of the anterior, oblique end in short 

 pieces and more rarely of both head and posterior end on the posterior 

 oblique surface (Fig. 25, D).^'' 



In the reconstitution of short pieces half, or less than half, the width of 

 the planarian body the head may develop on the angle between anterior 

 and median cut surfaces or entirely on the median, instead of on the 

 anterior, surface. The longitudinal axis thus determined is oblique or 

 vertical to the original anteroposterior axis. In these heads the eyespot 

 of the side nearest the original anterior end often appears first, and the 

 whole head may be asymmetrical in earlier stages.'* 



VISIBLE EVIDENCES OF PATTERN IN EARLY EMBRYONIC DEVELOPMENT 



The earliest indications of embryonic order and pattern are usually 

 regional differentials or gradients of one kind or another — of amount of 

 yolk, visible character and staining of the cytoplasm of the egg or cleav- 

 age stages, rate of cell division, size of cells, rate of growth, rate of dif- 

 ferentiation, etc. Some of these evidences of pattern — ^yolk gradients, for 

 example — are present in undivided eggs of some species; others appear 

 during cleavage or later; still others, only as particular organ systems 

 begin visible development. Very commonly pattern is first evident along 

 what becomes the polar axis and becomes evident in other directions 

 corresponding to other axes as development proceeds. However, some 

 eggs exhibit, at or before the beginning of development, more or less 

 definite regional differentiations in the cytoplasm which are, or become, 

 features of embryonic pattern. So-called "mosaic development" furnishes 

 numerous examples of this type of pattern (see chap. xiv). Such eggs may 

 be, and now usually are, regarded as having already undergone a greater 

 or less degree of regional cytoplasmic differentiation which becomes the 

 basic of embryonic differentiations. Since these differentiations are, or be- 

 come, definitely related to axiate embryonic pattern and since they occur 

 only in certain species, they probably represent derivatives, effects, ex- 

 pressions of pattern, rather than the primary pattern itself. But whatever 

 the condition of the egg cytoplasm at the beginning of development, 



" These oblique reconstitutions in planarians have been repeatedly described. See, e.g., 

 Morgan, 1900, 1901c, 19046,- Bardeen, 1902; Rand and Boyden, 1913. They have been used 

 by the writer as class experiments for many years. 



'* Morgan, 1900, 1901c, 1904&; Child, 1915c, p. 164; Olmsted, 1918; Beyer and Child, 1930. 

 See also pp. 365-66. 



