THE PYCNOGONIDS. 23 



the latter may be explained by considering it an abbreviation 

 of the type represented by Phoxichilidium. An exact com- 

 parison of the segmentation of the two forms would be of 

 interest; but, in order that such a comparison should be of 

 value, the exact orientation of the segmentation plane would be 

 essential, and such observations are wanting. 



It would seem a priori most probable that the first planes in 

 each must correspond, and that the unequal segmentation of the 

 e^g of Pallene has been caused by the greater accumulation of 

 yolk in that part of the egg which corresponds to the macromeres. 

 It is also probable that the smaller segments — the micromeres — 

 correspond either to the anterior or to the ventral part of the 

 embryo(?). Which of these is correct it is difficult to say. The 

 embryo differentiates earlier in what corresponds to the anterior 

 region of the adult than over the whole ventral surface, which 

 suggests that the smaller cells may have adapted themselves to 

 this early differentiation ; but it seems equally possible that this 

 differentiation may be due to phylogenetic laws in this particular 

 case rather than to any mechanical connection with the micro- 

 mere differentiation. So that for the present the question must 

 remain unsettled until by actual experiment (which would not 

 be difficult) the orientation of the segmentation planes be 

 determined. 



There is an observation on the segmentation of the egg of 

 Pallene brevirostris by Hoek, which was mentioned in the pre- 

 vious description of segmentation of Pallene empusa. He says : 

 " Le fractionnement commence par le fractionnement du noyau, 

 et seulement apris que quartre noyau sont formes, un premier 

 fractionnement devise l'oeuf en une partie plus grand et une 

 autre beaucoup plus petite. Chaque partie contient deux 

 noyaux qui dans le plus petit segment sont plus rapproches 

 1'une de 1' autre que dans 1' autre segment." 



I cannot believe this a correct observation in the light of what 

 I have observed over and over again in Pallene empusa. There 

 is a stage which corresponds exactly with the description, but 

 the egg itself has previously divided into two with a single 

 nucleus in each segment, and subsequently each of these nuclei 

 divides into two just before the segment itself divides, and at the 

 same time the first furrow becomes more distinct again as the 



