640 HAROLD HEATH, 



divides into an anterior and posterior instead of into a right and left 

 half. In this case therefore there is an anachronism or delay in the 

 normal first cleavage. Also the same author has shown that in the 

 frog certain structures may appear in the entoderm or mesoderm at 

 different times with reference to the same stage in the differentiation 

 of the medullary canal. 



In the early development of Ischnochiton the first type of ana- 

 chronism has never been observed. In the later stages there are 

 frequent delays in certain cleavages with reference to others but they 

 are by no means so fundamental as the one described by Roux. 



Also in the development certain cleavages and shifting of cells 

 and the appearance of Anlagen of various organs may occur at 

 different times with reference to the closure of the blastopore or its 

 later migration, but it is very difficult to judge the extent of the 

 correlation as the form of the various layers or organs is not always 

 a safe criterion for determining the amount of differentiation which 

 is present. 



In the above cases the anachronism is sporadic, occurring very 

 infrequently and due to influences as yet beyond our knowledge. Such 

 cases may be said to be ontogenetic anachronisms in contradistinction 

 to another species occurring constantly in the development of Ischno- 

 chiton which may be termed provisionally phylogenetic anachronism. 

 In this case also the ultimate factors producing the omission of 

 cleavage are as much unknown as in the other types, but some of 

 its superficial features are subject, I believe, to a more accurate 

 analysis. 



In other sections of this paper it has been urged that the radial 

 condition of the embryo which precedes the bilateral is the repetition 

 of the phylogenetic history in which a radial ancestor preceded the 

 bilateral. This ontogenetic record however is not perfect; precocious 

 segregation among other factors has wrought deep seated changes 

 bringing about an early appearance of bilateral cleavages, and in many 

 cases it is clearly evident that these latter are ' modifications of the 

 radial. And I hold that it is not an accident that a secondary trocho- 

 blast forms in the posterior quadrant in the same way and with the 

 same relations as in the other quadrants. It bears evidence of the 

 fact that originally secondary trochoblasts arose in each 

 quadrant and were functional; and also that the shifting 

 from its original position into a region below the level 

 of the prototroch, thereby producing the well known 



