CRANGON VULGARIS. 133 



would seem, in Neophalax. 1 In the mite, Tetranychus 

 (Claparede, '68 ) the process has gone a step farther, for 

 here the segmentation nucleus reaches the surface before 

 it divides. In this case the segmentation is necessarily 

 superficial, but it takes but a very short time 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 pi. 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 time. 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- 

 lar 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 Tetrauychus, it is but 

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

 the segmentation nucleus reaches the surface before or 

 soon after segmentation begins, but the resulting 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 hours after oviposition ... a clear space makes its ap- 

 pearance at the surface of the egg and gradually increases until it has attained the 

 breadth of the future blastoderm" Patten ('84, p. 5G3). 



