CHAPTER VI 



SPIRALLY CLEAVING EGGS 



THE SPIRAL type of cleavage is sufficiently definite to allow one to recog- 

 nise the existence of a common pattern in several groups of animals which 

 are not otherwise very closely related (molluscs except cephalopods, 

 nemerteans, platyhelminths, annelids); but overlying the basic similari- 

 ties there are very many variations in detail, both in the early cleavages 

 and in the types of larvae which are produced by early development. To 

 keep within the limits of space available here, it will be necessary to 

 attempt a ruthless simplification. 



The cleavage begins by two more or less vertical divisions, cutting the 

 egg into four blastomeres, which are typically somewhat twisted in 

 relation to each other. These four are conventionally known as A, B, C, 

 and D. hi the next few cleavages, each of these cells remains as a fairly large 

 macromere, and gives off a succession of smaller micromeres, the first 

 group of which are known as la, lb, ic, id, the second group as 2a, 2b, 

 2c, 2d and so on. The cleavage spindles by which these divisions occur 

 do not lie either vertically or horizontally, but at some angle between; 

 and if the spindles are tilted to the left of the vertical in the formation of 

 the first group, they will be tilted to the right for the next group. There is in 

 fact an alternation from one tilt to the other. While the macromeres are 

 giving off new rings of micromeres in this way, the already formed 

 micromeres continue to divide in the normal way, again with their spindles 

 tilted like those of the macromeres (Fig. 4.2, p. 61). 



This regular pattern usually continues for four division-cycles. The sub- 

 sequent fate of each of these cells has been followed in detail, and we know 

 exactly what organs each will form in the later embryo. There is no need 

 to go into great detail about this here. Roughly, the macromeres form 

 endoderm, and most of the micromeres ectoderm; but there are two spec- 

 ial micromeres which produce the mesoderm, and which we shall fmd 

 play a peculiarly important role in the mechanics of development. These 

 are the cells id and 4J, both of which, as their designation indicates, are 

 ultimately derived from the D macromere. 



Events after the initial cleavage vary a great deal in different groups. 

 In many species a larva is formed; and this is usually some variety of the 

 'trochophore' type (Fig. 6.1). In the development of this, the ectodermal 

 micromeres grow down over the macromeres, which bend inwards to form 



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