MUSEUM OF COMPARATIVE ZOOLOGY. Ill 



at the centre of the yolk in the first condition of tne egg. It there be- 

 comes more dense and divides. Of the two masses of unequal size, 

 " Toujours," he says, " une des deux pr^sente des proportions conside- 

 rables par rapport h I'autre." In fig. 3 PL XI., to which he refers for 

 this phenomenon, this difference in size of the two is not well shown, 

 and the condensed central region is not separated from the superficial by 

 as strong a line of demarkation as in Ophiopholis. It is supposed that 

 the condensed central part which is referred to in his description is the 

 same as the slightly opaque or greenish centre of the Ophiopholis egg, 

 but such an interpretation is open to doubt. The transparent superficial 

 layer is not homogeneous throughout. At one pole on the outer surface 

 of the transparent layer of the yolk, still fastened to it or not sepa- 

 rated from the yolk cells, a single globule was observed, PI. I. fig. 2, c?, 

 possibly in the process of forming. This globule forms a slight elevation 

 on the surface of the transparent layer, and a corresponding conical ele- 

 vation was observed under it on the denser part of the vitellus.* Later 

 in time a globule separated from the yolk was observed, and in later 

 stages of development an additional globule is formed, PI. II. fig. 3. 

 The largest number of free polar globules observed was two. Polar glob- 

 ules are not figured or mentioned in Ophiothrix by Apostolides. He 

 speaks of them, however, in Amphiura. The superficial layer of the 

 yolk appears to surround both cells, and in the contiguous surfaces of 

 the two cells this layer is undivided, corresponding in its position with 

 the plane of the first cleavage, PI. I. fig. 3. This coincidence causes the 

 two cells of the 2-cell stage to appear separated by a transparent layer, 

 which at the same time unites them,t Fig. 3, 1 cl. pi. 



The formation of the 2-ceU stage does not occur immediately after tht 

 sperm is added to the glass containing the ova. As in Echinarachnius a 

 considerable time elapses after the mixture of the two elements before 

 ^jhe formation of a 2-cell stage. The indications are that the first changes 

 go on more rapidly in Ophiopholis than in Ophiothrix, as observed by 



• Thi8 conical elevation may be connecieu with the " Dotterhiigel " of Fol and 

 others. It has, however, no existence on the outer surface of the plasmic or corti- 

 cal layer. A more acute histological examination of the single globule on the sur- 

 face of the latter above the conical elevation is necessary before it can be stated 

 whether it is a polar globule or a spermatozoon. I regard it as the first polar 

 globule. 



t A characteristic connection of the two blastomeres is mentioned in the egg of 

 Ophiophragma by Professor Nachtrieb. Whether it has a likeness near or remote 

 to this condition in Ophiopholis, it is impossible for me to say. From hit short 

 description I am unable to compare the two genera in this regard. 



