The Development of Ischnocbitou. 639 



(Cephalopods) bilateral cleavages may arise from the beginning and 

 no trace of the radial exist. 



Where this bilateral condition appears in the embryo it is usually 

 in connection with bilateral cleavages and I heartily agree with Conklin 

 when he says "the bilaterality of cleavage is only an early appearance 

 of the final bilaterality with which it is directly continuous". But 

 I believe that when he says "In several cases these radial 

 structures" (speaking of the radial symmetry of the quartettes, 

 trochoblasts, apical rosette( Molluscan cross etc.) "seem to belong 

 to the same category as the radial structures of the 

 trochophore larva, and I believe that they are to be 

 explained as a foreshadowing of larval characters, 

 just as bilateral cleavages are usually attributed to a 

 precocious development of adult characters", it holds 

 good not for both radial and bilateral characters but simply for the 

 bilateral. Rather may it be said that bilateral prelarval stages of 

 Annelids and Molluscs may be held to be due to the similarity of 

 their larvae, but considering the radial form of the embryo I believe 

 that the problem has been attacked from the wrong side. I believe 

 rather that the radial symmetry found in the trochophore has per- 

 sisted in spite of the bilateral condition, and that it belongs to a 

 period in the phylogenetic development of the organism which preceded 

 the bilateral. Therefore just as bilateral cleavages are due 

 to a bilateral condition of the trochophore, so Ibelieve 

 radial cleavages are due to the influence of the radial 

 symmetry of the ancestor of the trochophore (cf. Lang). 



While the formation of a ventral surface gave rise to a bilateral 

 form it does not follow that bilateral cleavages arose at that time. 

 On the other hand it appears that at least in the ectoderm and 

 entoderm these have arisen at a later period. And neither does it 

 follow that in the early history of the radial ancestor the cleavages 

 were spiral. Rather it appears to me that, as in the case of bilateral 

 cleavages, the spiral divisions gave a greater directness to the de- 

 velopment, and arose from a saving of energy which takes place, as 

 Conklin holds, whenever precocious segregation is present. 



4. Anachronism of Cleavage. 

 In the frog's egg Roux ('95) has shown that the cleavage, which 

 normally appears as the second in the development, may occur first. 

 That is, the egg with reference to the embryo, to which it gives rise, 



