376 PATTERNS AND PROBLEMS OF DEVELOPMENT 



species. It has often been noted, sometimes as something remarkable and 

 difficult to account for, that embryonic stages of certain forms appear 

 almost completely incapable of reconstitution, while adults of the same 

 species have high reconstitutional capacity. This is the case in many an- 

 nelids. The only difficulty here is the failure to recognize that the egg 

 may have a higher or more stable regional differentiation than different 

 levels of the adult body and that much of the egg differentiation may be 

 lost during development. In the annelid the embryonic differentiations 

 give rise mostly to larval parts and to head regions; it has never been 

 shown that the cells from which postcephalic regions develop possess any 

 such differentiation. In fact, there is evidence from their reconstitution 

 that they do not. 



With the progress of experimental investigation and analysis it appears 

 increasingly evident that essential features of developmental pattern in 

 many eggs and early embryonic stages are fundamentally similar to fea- 

 tures of pattern in adults of many of the simpler organisms, as far as 

 spatial order and relations are concerned. If this is true, reconstitutional 

 development in adult material, being usually more accessible to experi- 

 mental analysis than that in parts of eggs and embryos and bringing us 

 nearer the beginnings of developmental pattern than the beginning of 

 embryonic development in many forms, should be regarded as an invalu- 

 able aid in throwing light on problems of embryonic development and 

 reconstitution. 



A few data concerning modifications of embryonic pattern in relation 

 to section and isolation and involving points of special interest are briefly 

 discussed. Alteration by section of the polar pattern in the egg of the 

 sea urchin Lytechinus variegatus has been reported. Using position of 

 polar bodies and micropyle for identification of the original polar axis, it 

 was found that "animal" and "vegetal" halves, meridional halves, and 

 halves resulting from section in random directions of unfertilized eggs 

 may, after fertilization, reconstitute into wholes. Pieces down to 1/20 of 

 the total egg volume, irrespective of the region concerned, may become 

 normal blastulae with primary mesenchyme ; micromere formation is not 

 necessary for formation of primary mesenchyme. Section of unfertilized 

 eggs of Strongylocentrotus piirpuratus, S. franciscanus, and of the starfish 

 Patina miniata is followed after fertilization by first and second cleavages 

 in a plane vertical to the plane of section ; and when plane of section can 

 be identified at the stage of gastrulation, invagination occurs on the sur- 

 face of section. Moreover, in Lytechinus micromeres form on the surface 



