INVERTEBRATES 673 



The divisions which form them are at right angles to the first 

 and second, which have produced the macromeres, and the 

 micromeres do not he directly above each other, but are displaced 

 alternately to right and left of the planes through the macro- 

 meres. All the micromeres of the first three quartets will form 

 ectoderm. The sixth division produces a fourth quartet, with 

 cells of unequal size. The macromeres, together with d^, d2, and 

 dg, form endoderm, while d4, and d4 alone, forms mesoderm. 

 Its removal therefore results in an embryo from which all meso- 

 dermal structures are lacking. Cleavage of this sort, where a par- 

 ticular tissue is linked to a particular cell in the early embryo, 

 is called determinate. In many species the protoplasm which will 

 form d4 can be recognised even in the e^g. 



0) V^ N^^/ ('■') 



Fig. 528. — Spiral cleavage. The embryos are seen from the animal pole. 



(i) Eight-cell stage ; A, B, C, D, the macromeres ; the micromeres stippled, (ii) Twenty-cell stage, showing 

 the macromeres ^, B, C, D, and the four quartets of micromeres, ai, ftj, ci, di, etc ; d^ (stippled) is the 

 cell which will give rise to the mesoblast. 



DIFFERENTIATION 



The process of development is inevitably compUcated ; the 

 individual begins as an egg, which is structurally a single cell, and 

 ends as a vertebrate or insect or whatever it may be. It is not 

 surprising therefore that, since they start from the same point, 

 embryos resemble each other more or less for some distance along 

 the road which they travel, and that the greater the final 

 resemblance of the adults, the longer does the similarity of the 

 embryos persist. Thus while resemblance between a vertebrate 

 and a mollusc is lost soon after cleavage, a 5j-day chick and a 

 13-day rabbit embryo are almost indistinguishable. For some 

 days more the rabbit can hardly be distinguished from other 

 mammals, then it becomes recognisable as a lagomorph (p. 484) 

 and finally as its own species. The sex of an animal may be 

 indistinguishable until long after it is fully grown, and even 

 then, as in many birds, only by its behaviour or by dissection. 

 Further, there may be some degree of resemblance between the 



