638 HAROLD HEATH, 



Molluscs diverged from the ancestral trochophore after it had become 

 bilateral. Therefore, theoretically, all the embryos in their ontogeny 

 should pass along the same developmental path up to the assumption 

 of the bilateral form. That this does not actually happen is owing 

 probably to secondary factors, such perhaps as the reflection of the 

 bilateral form upon the earlier stages (precocious segregation), ac- 

 cumulation of yolk, etc., which tend constantly to modify this ancestral 

 condition. Hence when bilateral cleavages appear in development 

 previous to the shifting of the blastopore they may be considered 

 secondary, and in many cases they show that they are such, exhibiting 

 unmistakable modifications from a radial type. And as has been 

 maintained above, it does not necessarily follow that these secondary 

 bilateral cleavages everywhere follow the same path, for even though 

 they have been moulded upon similar radial forms they tend to depart 

 from a common type. 



3. Forms of Cleavage. 



Growing out of the above considerations some points arise relating 

 to the significance of radial and bilateral types of cleavage. As I 

 have said above I believe that the early radial condition of the embryo 

 is of fundamental significance; that it represents an early primitive 

 condition phylogenetically when the ancestor of the trochophore was 

 a radially symmetrical organism. The trochophore as we recognize 

 it today has been modified from this condition owing to the formation 

 of a ventral surface and correlated shifting of the mouth, and one 

 of its most distinctive features is its bilateral form. 



In development the organism passes from a radial to a bilateral 

 condition, a process which in Ischnochiton is of greater duration than 

 in some Annelids (e. g. Nereis, AmpJiitrife, Arenicola) or in Uni(y 

 among the Lamellibranchs. That is to say, Ischnochiton longer retains 

 its primitive characters than these forms in which the radial structure 

 is soon replaced by the secondary bilateral. And in those forms 

 where precocious segregation is most pronounced the radial symmetry 

 is as a rule proportionately decreased, from which it follows that the 

 reflection of larval stages upon the early stages does 

 not produce radial symmetry but tends to destroy it. 

 There is a warfare, so to speak, a contest between the radial and 

 bilateral condition " wherein the latter appears to be slowly replacing 

 the former, and it has even gone so far as to produce marked effects 

 in some cases at the first cleavage, and theoretically if not actually 



