MUSEUM OF COMPARATIVE ZOOLOGY. 123 



elements were observed crowded upon the outer surface of the viscid 

 layer, with heads partially buried in it. None were observed to pene- 

 trate to the yolk. The egg was seen to be slightly jerked about, possi- 

 bly by the combined movement of the many spermatozoa on its surface. 

 In no case was the movement very great. No polar globules were 

 observed.* 



Cleavage. 



The segmentation of the egg of Echinaracbnius is regular, and the 

 first formed segment spheres are of the same size. After the formation 

 of the 8-cell stage from the 4-cell an inequality in size of the blastomeres 

 is noticed. As in Strongylocentrotus one of the first changes after the 

 disappearance of the nucleus is the drawing away of the yolk from the 

 shell. From an hour to two hours after the ova and sperm have been 

 artificially brought together, the first cleavage furrow, p, is noticed 

 encircling the egg. 



In some eggs this furrow, PI. III. fig. 1, is limited to one pole, and 

 the indentation gradually deepens until the e^,g is divided into two 

 hemispheres connected at the pole opposite that at which the fur- 

 row first appears. Folds which recall similar plications observed by 

 Metschnikoff in the Epibulia egg, and by myself in the egg of Agalma, 

 appear on each side of this primitive furrow, PI. III. fig. 2. These 

 wrinkles are supposed to be the " Faltenkranzen." This method of seg- 

 mentation reminds me of what we have in the egg of the Sinhonophore. 

 It was not traced beyond the 2-cell stage. 



In most cases the primitive furrow is not limited to one pole, but 

 girts the ovum. Four cells were, however, observed in a 4-cell stage, in 

 each of which the furrow, which is to form a new cleavage plane, is 

 limited to one pole of the cell. PI. III. figs. 6, 7. 



In those ova in which the primitive furrow girts the eg^,, the con- 

 striction deepens uniformly on all sides, until the ovum is divided into 

 two equal spheres, PI. II. fig. 3, united by flat faces with each other. 

 In each of the two cells a nucleus can be seen. The blastomeres of the 

 2-cell stage are never seen widely separated from each other. 



* According to Nachtrieb no polar globules were observed by him in the closely 

 allied genus Mellita. I suspect, as is well known in some other Echinoids, that 

 the polar globules are formed while the egg is in the ovary. 



t More than one method of cleavage has been observed in the Oyster by Brooks, 

 and in Reniila by Wilson. It is not improbable that the segmentation of Echin- 

 aracbnius mentioned above is a second kind of cleavage. 



