CHAPTER XIII 



CERTAIN EMBRYONIC RECONSTITUTIONS IN RELATION 

 TO PRE-EXISTING PATTERN 



MANY of the regional differences in reconstitution of isolated 

 embryonic parts are evidently expressions of the axiate pat- 

 tern already present and therefore of interest in relation to 

 the problem of its nature. This chapter is concerned with some of these, 

 particularly in forms with relatively high capacity for reconstitution in 

 earlier stages. 



COELENTERATES 



Although embryonic stages and planulae of this group should be in- 

 teresting in this connection, experimental data are few. According to Zoja 

 (1895), hydroids may develop from 1/2, 1/4, 1/8, and even from 1/16 

 blastomeres of certain forms, but nothing is known concerning differences 

 in capacity in relation to the original pattern. However, if 1/8 and 1/16 

 blastomeres can develop into planulae that give rise to hydroids, there 

 is probably little or no regional differentiation in the pattern. How a 1/16 

 blastomere attains the pattern of the whole is an interesting question. 

 In view of the instability of polarity in the embryo and planula of Phia- 

 lidium (pp. 167, 425), it seems possible that in some of these isolated blas- 

 tomeres a new polar pattern may be determined by the differential be- 

 tween free surface and surface in contact. 



In some other forms pieces of undivided eggs above a certain size and 

 1/2 and 1/4 blastomeres can become wholes, but 1/8 blastomeres result- 

 ing from equatorial division become abnormal larvae or show little de- 

 velopment.' Pieces of some hydrozoan planulae give rise to hydroids 

 (Maas, Child), but pieces of planulae of the anthozoan Renilla do not 

 develop beyond the swimming stage. These fragmentary data throw little 

 light on the question of the original pattern in these forms. Limitation 

 or absence of reconstitution in 1/8 blastomeres or in planula pieces sug- 

 gests some degree of polar differentiation but may perhaps be due to scale 

 of polar organization too large for the blastomere or piece rather than to 



I Maas, 1901, 1905, 1908; E. B. Wilson, 19036; Conklin, 1908. 



504 



