,V)2 IIAK'OI.D H. PI,<>r<,H. 



SUMMARY AND Coxri.rsioxs. 



1. The developmental history of more than three hundred 

 blastomeres ot Arbacin and Rchinarachnins isolated in the two 

 cell stage and followed individually tor at least tour days is 

 recorded. 



2. About lorty per cent, ot these blastomeres form ciliated 

 arv;e with a complete tripartite gut, but these larva- fall in three 

 classes with respect to the skeleton ; (a) those which never develop 

 a skeleton, (b) those in which the skeleton is defective or partial, 

 (c) those with a complete pluteus skeleton of one half size. 



3. When the pairs of blastomeres from the same egg both of 

 which developed to the larval stage are considered alone, it is 

 found that they fall into three significant groups: (a) a very 

 small number ot pairs from which developed two plutei with per- 

 fect skeletons of one half size, (b), a much larger number from 

 which developed two plutei each with a skeleton which was in- 

 complete, (c), a group which formed one perfect pluteus and one 

 larva without a skeleton. 



4. These facts appear to prove that there is already localized 

 at the time of the first cleavage in the eggs of A rbacia and Echinar- 

 achnius a layer of skeleton forming material at right angles to 

 the primary egg axis, and in the vegetative half of the egg, which 

 may be cut at any angle by the first cleavage plane and unequally 

 distributed to the first two blastomeres. 



5. The eggs of these two sea-urchins are thus shown to be (to 

 use the words of Professor Wilson) "no more isotropic than those 

 of the mollusk or annelid." It seems probable that this is true 

 tor other sea-urchin eggs as well. 



6. In some cases the two incomplete skeletons of a pair of 

 blastomeres trom the same egg are complementary. This sug- 

 gest > turther a certain amount ot bilateral organization ot the 

 skeleton forming material already localized in the egg at the time 

 ot the tirst cleavage. Thi- idea receives suppon trom the obser- 

 vations of Runnstrom on Paracentrotiis. 



I ! 1 1 K.\ II KK CITKD 

 Boven, Th. 



'01 I >ic I'oluiitat von Ovocyto, lu. un.l Larvij >[<- . i-titmtm liritiits. 



'i"l Jalir. Bd. XIV. 



