554 PATTERNS AND PROBLEMS OF DEVELOPMENT 



in annelids and mollusks, presents the best case for cell homology in these 

 groups; but even here there are difficulties. According to Schleip, it gives 

 rise to only one of the mesodermal bands in Clepsine, the other being 

 formed from 4D, which is entodermal in other forms. Moreover, 4d is 

 apparently entodermal in polyclads. Actually there are very few, if any, 

 exact cell homologies in the groups with spiral cleavage. In order to apply 

 the concept of homology to the cells, we must, in any case, regard certain 

 cleavages as accomplishing segregation and others as nonsegregating. Vis- 

 ibly different regions are often more or less exactly segregated in different 

 cells, but the centrifuge experiments show that this segregation has little 

 or no significance for development in most cases. If the principle of homol- 

 ogy is applicable to egg cytoplasm, the homologies seem to be, at best, 

 regional; and their more or less exact coincidence with cell boundaries 

 appears to be an incidental or secondary result of regional cytoplasmic 

 differences of some sort rather than fundamentally significant for develop- 

 ment. 



A particularly interesting feature of development with spiral cleavage 

 is the gradual appearance of bilateral pattern in the cleavage pattern and 

 its inexact character in early stages of most forms. Different species and 

 different regions of the embryos provide various examples. The annelid 

 first and second somatoblasts, 2d and 4d, and in Clepsine also 4D, are 

 cases in point. The cell plate (somatic plate) resulting from divisions of 

 2d in polychetes extends laterally on each side from the dorsal region and 

 gradually acquires a roughly bilateral form (Fig. 172, F), but up to an 

 advanced stage most of the cleavages in it have not been bilateral. More- 

 over, the median plane of the two mesoblasts does not coincide with that 

 of the ectoderm of the somatic plate (Fig. 172, F), but in later stages 

 coincidence is gradually attained. In Clepsine bilateral pattern appears 

 earlier in descendants of 2d (Fig. 173, D); but even after the germ bands 

 begin to form, the arrangement of the teloblasts is not necessarily com- 

 pletely bilateral, and the median planes indicated by the ectodermal telo- 

 blasts and by the two mesoblasts are farther from coincidence than in 

 polychetes (Fig. 173, E). 



The pattern resulting from spiral cleavage is essentially quadriradial 

 in polyclads, in nemerteans, as far as known, and in some annelids and 

 mollusks. In this pattern there gradually appears a dorsiventral pattern 

 with bilateral symmetry or, in the gasteropods, asymmetry. These facts, 

 quite apart from experimental data, raise the question whether this form 

 of development is as completely a mosaic as has been believed. Is it pos- 



