246 BULLETIN OF THE 



Cleavage. 



First Cleavage Fiirroiv. — Gegenbaur,* who says that he observed the 

 segmentation of the ovum of the genera Agalmopsis, Fhi/soj^hora, Fors- 

 kalia, Hippopodms, and Diphyes, states that the whole process of seg- 

 mentation is finished in from twenty-four to tliirty-six hours. Haeckelf 

 says that in Physophora, Crystallodes, and Athoryhia the segmentation 

 is finished at the end of the second day. Metschnikoff does not state 

 the exact limit of time when the segmentation is finished, although 

 from the age of the youngest larva of Agalma which he figures I should 

 judge that the segmentation was accomplished in the second day. All 

 recorded observations on Siphonophore eggs point to the conclusion that 

 the cleavage is wholly completed before the beginning of the third day 

 after fecundation. 



jNIy first specimen of Agalma was captured on August Gth, at 

 noon, and before the morning of August 8th it had laid eggs which 

 were in the same stage as that figured by Metschnikoff on the fourth 

 day. In other words, a little over a day and a half after the Agal- 

 mata were placed in tlie aquarium, eggs from them had segmented and 

 had formed the two layers described Vjy Metschnikoff in the changes 

 of the fourth day. My observations ai^e thus at variance with those of 

 Gegenbaur, Haeckel, and Metschnikoff. What is the meaning of the 

 discrepancy^ Looking over my notes in vain to find an error in this 

 particular, it has seemed possible that errors of observation have crept in 

 for the reason that individual eggs have not been followed through their 

 consequent stages. An Agalma in captivity will mature its eggs at dif- 

 ferent times, so that at the end of the fifth day segmented eggs in com- 

 pany with those which are far along in the development of the primitive 

 hydrophyllium may be picked out of the same water. From the nature 

 of the case, unless individual eggs are isolated and the time of their 

 fecundation recorded, it is impossible to know the age of any specified 

 stage. 



The first change which takes place in the spherical egg after it has 

 left the gonophore is the formation of the primary cleavage furrow, pr. 

 At one ])olc of the ovum (PL I. fig. 7) an indentation appears in the 

 form of a furrow on the surface of the e<2'g. Altliouirh I have not ob- 

 served at the outset tlie exact relationship of this furrow to the rosy 

 pole, I have seen that later, after the first plane of cleavage has been 



* Op. cit., p. 50. 



+ Op. cit,, for Physophora, p. 19 ; for Crystallodes, p. 51 ; for Athoryhia, p. 89. 



