376 HAROLD H. PLOUGH. 



the left side. Such larvae never become normal. Unfortunately 

 data are not given to show how many blastomeres developed in 

 this manner, nor what happened to the other member of the pair 

 when one gave this result. Apart from this, however, Runn- 

 strom's conclusion seems justified that there was an indication 

 of a bilateral organization of skeleton forming material in the 

 undivided egg of Paracentrotus. I have found a number of cases 

 of the same sort in Arbacia and Echinarachnius , suggesting that 

 in these eggs also there is already some bilaterality of skeleton 

 forming material in the undivided egg. 



The bearings of the present investigation may be shown 

 with greater clearness by reference to a diagram. Several of 

 the earlier writers have indicated that material which goes to 

 form the larval skeleton seems to be localized in the four micro- 

 meres cut off from the lower quartette of cells at the fourth 

 cleavage (sixteen cell stage). Von Ubisch has apparently con- 

 firmed this by staining these cells and finding that the material 

 derived from them formed the larval mesenchyme. On Boveri's 

 view this skeleton forming substance may be thought of as strati- 

 fied at right angles to the initial egg axis in the undivided egg. 

 That this is probably the case in Arbacia is indicated by the recent 

 work of Harnly (1926). He reported experiments in which fertil- 

 ized eggs of Arbacia were cut in two before the first cleavage, and 

 he found that the nucleated portions segmented either as the 

 dorsal, lateral or ventral hemisphere as determined by the 

 presence or absence of micromeres. This result shows, he be- 

 lieves, that there is a localized equatorial area of micromere 

 forming substance in the undivided egg below the nucleus and in 

 the vegetative half of the egg. The later history of these frag- 

 ments is not described, but we may assume that the material 

 which will form the skeleton is located in the egg as suggested. 

 According to Von Ubisch's evidence the first cleavage plane may 

 cut the egg as shown in Fig. I. A and C indicate the extremes, 

 and B one of the possible planes between. According to this plan 

 we should expect that each blastomere of A, if isolated would 

 develop as a whole larva of one half size. If those of C are separ- 

 ated, however, the upper blastomere would lack the skeletal 

 material, and one complete larva and one lacking a skeleton 



