494 



SCIENCE. 



[N. S. Vol. XVII. No. 430 



that lies just below the dark groove in the 

 middle of the slate-colored area continues 

 into the egg along the dorsal wall of the 

 archenteron. 



These embryos do not appear to be able 

 to develop much beyond this stage, although 

 they may remain alive for several days 

 longer in the solutions. The interpreta- 

 tion of this peculiar method of develop- 

 ment seems to be as follows: The cells of 

 the upper hemisphere appear to have been 

 prevented from growing down at the sides, 

 and, after the blastopore has been formed, 

 from covering over the lower hemisphere. 

 The medullary plate develops from the 

 inner wall of the cap of ectoderm that has 

 been turned into the interior of the egg. 

 As this upper region sinks in, the surface 

 yolk cells below the equator of the egg are 

 drawn upwards, as has been said, and pro- 

 duce the slate-colored band. They may be 

 supposed to represent, in a general way, the 

 dorsal wall of the archenteron of the nor- 

 mal egg, which is now spread out on the 

 surface of the egg. This comparison needs, 

 however, several important limitations, 

 which I can not enter into here. The rest 

 of the archenteron is represented by the 

 long but very narrow tube leading in- 

 wards from the blastopore. Thus the em- 

 bryo is, in a sense, inverted, the nervous 

 system being in the interior of the egg, 

 and yolk cells almost completely covering 

 the surface. The result is due in all prob- 

 ability, in part, to changes in the osmotic 

 conditions in the egg. I hope soon to 

 describe, with figures, these embryos, as 

 well as other kinds produced in the same 

 solutions. 



Experiments on the Origin of the Cleavage 

 Centrosomes: E. G. Conklin, University 

 of Pennsylvania. (To be published in 

 the Biological Bulletin.) 



On the Erosion of the Shell of Littorina 



litorea: R. P. Bigelow and Eleanor P. 



Eathbun. 



The investigations of Morse and Ganong 

 have shown that Littorina litorea has be- 

 come established on our coast within the 

 last half century, and Bumpus has made 

 a statistical study of the species, from 

 which important conclusions were drawn 

 as to the changes of type and variability 

 resulting from this change of environment. 



Therefore, this species seemed to be a 

 favorable one upon which to make a de- 

 termination of the present rate and direc- 

 tion of natural selection. But it seemed 

 wise to determine first how far erosion 

 might tend to falsify the results. 



Sections of fresh shells were made by 

 the method used in sectioning minerals, 

 and the chief results are given in the fol- 

 lowing table. 



The section shows that erosion has begun 

 in stage 1; but, as that is the smallest shell 

 that it was possible to cut, the erosion is 

 assumed to be zero for purpose of com- 

 parison with the later stages. 



These observations are not sufficiently 

 numerous to have a statistical value, but 

 they are sufficient to show that the factor 

 of erosion must be considered before any 

 conclusions can be drawn from a statistical 

 study of the dimensions of shells of this 

 species, and to suggest that it would be 

 well to make sections and study carefully 

 the extent of erosion before publishing the 

 results of measurements of any other gas- 

 teropod shell. 



