CRANGON VULGARIS. 133 



would seem, in Neophalax.^ In the mite, Tetranychus 

 (Claparecle, '68) the process has gone a step fartlier, for 

 here the seo:nientation nucleus reaches the surface befove 

 it divides. In this case the segmentation is necessarily 

 superficial, but it takes but a veiy short tinie to have the 

 whole surface of the egg covered with nuclei, a process 

 which is apparently completed before the appearance of 

 anything like a germinal groove. In this connection, Clap- 

 arede's pl. XL, figs. 1, 2 and 3, are instructive, for they 

 clearly show us that a superficial segmentation in the Ar- 

 thropoda is necessarily meroblastic, though here this con- 

 dition lasts but a short tinie. A superficial segmentation 

 demands that both nucleus and protoplasm be placed 

 practically at the surface of the yolk ; in other words, an 

 egg which cannot be distinguished from one of the regu- 

 lär meroblastic type. When segmentation commences, it 

 must necessarily begin at the pole occupied by the nucleus ; 

 and, for at least the first few divisions, proceed most 

 rapidly in that region, the result being a meroblastic seg- 

 mentation, which cannot be defined as distinct from that 

 occurring in Cephalopods, Elasmobranchs, Sauropsida, etc. 

 It is certainly superficial, but superficial exactly in the same 

 way as in those forms mentioned which have never been 

 classed in the category of " centrolecithal eggs." 



From the condition which occurs in Tetranychus, it is but 

 a step to that occurring in Oniscus, Scorpio, etc. In these 

 the segmentation nucleus reaches the surfiice before or 

 soon after seo^mentation be2:ins, but the resultino^ blasto- 

 derm spreads more slowly over the yolk than in the mite 

 just mentioned, differentiation of the germ layers taking 

 place before the blastoderm covers half the yolk. Gradu- 

 ally, however, the blastoderm completely Covers the yolk. 



1 "Ten or twelve houvs after ovipositiou . . . a clear space makes its ap- 

 pearance at the surface of tlic egg and gradually increases until ifc has attaincd the 

 breadlh of the future blastoderm" — Patten ('84, p. fjüS). 



