14 



JV/LSON. 



In the typical development^ of all the forms in question — 

 polyclades, annelids, gasteropods, lamellibranchs — the egg first 

 divides into four quadrants. From these at least three, and 

 sometimes four or five regular quartets of cells — usually smaller, 

 and hence designated as *' micromeres " — are successively pro- 

 duced by more or less unequal and oblique cleavages toward 



Fig. 5. Diagram showing the typical arrangement of the micromere-quartets in 

 polyclades, annelids and mollusks (their secondary divisions being omitted). 

 A., from the upper pole. B, diagram of the typical history of the posterior quad- 

 rant of an annelid or gasteropod embryo ; ectoblast is derived from the vmshaded 

 cells (i, 2, 3), the mesoblast-bands from the dotted cell (4), ectoblast from the 

 lined cells (5, Z>). 



the upper pole (diagram, Fig. 5). These quartets are dis- 

 placed according to a definite law, the first being rotated, as it 

 were, towards the right (clockwise), the second towards the 

 left (anti -clockwise), the third to the right, and so on in regular 

 alternation.^ The secondary divisions of these micromeres also 



1 There are some well-determined exceptions to this mode of cleavage, and at least 

 one of these — the case oi Polychccnis, as described by Gardiner, 1895 — is apparently 

 irreducible to it. 



2 The reversal of the direction of displacement in the sinistral gasteropods, dis- 

 covered by Crampton, is an exception which emphasizes the rule. 



