386 HAROLD H. PLOUGH. 



A study of the remaining columns in the table reveals three 

 fairly striking facts. First, the number of pairs of blastomeres 

 each of which form complete larvae is very small indeed. I have 

 found only one out of eighteen eggs which gave this result in 

 Arbacia, and none out of nine in the sand dollar, although the one 

 recorded in column (6) showed only a slight defect in one arm of 

 one of the larvae. Second, among such pairs larvae with defective 

 skeletons are fairly common, but occur most often in pairs, 

 column (5). Probably the error shown by column (2) has pre- 

 vented some of those listed in column (4) from appearing in (5). 

 Third, there is a fairly large group of pairs of which one member 

 formed one complete half size pluteus and the other a larva with 

 no skeleton, column (3). 



For the purpose of our analysis then there would appear to be 

 three significant groups of eggs. While the actual numbers are 

 small they form a selected group from a very much larger number, 

 which as shown above tends to suggest the same result. The 

 first group consists of the small number of eggs each of whose 

 blastomeres formed perfect plutei, the second of eggs each 

 blastomere of which formed an incomplete pluteus, and the third 

 of eggs one blastomere of which formed a perfect pluteus and the 

 other a larva with no skeleton. It is obvious that these three 

 main groups correspond in general with the three possible direc- 

 tions of the first cleavage plane with reference to the initial egg 

 axis as suggested by the work of Von Ubisch, and as indicated in 

 diagrams A, B, and C in Fig. i. This interpretation is noted in 

 the last line oi Table II. Not only are these three groups found, 

 but in the sand dollar eggs at least the percentages of eggs which 

 fall in them correspond with expectation. The second group 

 should be the largest, and the first the smallest, as in fact they are. 



DEFECTIVE PLUTEI AND THE EVIDENCE FOR BILATERALITY OF 

 SKELETON-FORMING MATERIAL. 



A study of the larvae with defective skeletons not only sub- 

 stantiates the facts stated above, but suggests in addition a 

 certain amount of bilateral organization of the skeleton-forming 

 material which is apparently already localized in the vegetative 

 half of the egg at right angles to the initial egg axis. All of the 



