26 BIOLOGICAL LECTURES. 



teropods, and lamellibranchs conforms to the typical mode of 

 development. In all these forms the &gg first divides into four 

 quadrants. From these at least three and sometimes four or 

 five quartets of cells — usually smaller, and hence designated 

 as inicromeres — are successively produced by more or less 

 unequal cleavages towards the upper pole. The arrangement 

 of these micromeres (Fig. i) is constant and highly character- 

 istic, the first quartet being more or less displaced, or, as it 

 were, rotated in a direction corresponding with the hands of a 

 watch (clockwise), the second in the opposite direction (anti- 

 clockwise), the third clockwise again, and so on, the spindles of 

 each division being at right angles to those of the preceding 

 and following. In the later subdivisions of the micromeres, 

 also, a most remarkable agreement has been observed ; but I 

 shall pass this over entirely in order to focus attention on the 

 broader features of the development. 



A large part of the work in cell-lineage during the past ten 

 years has been devoted to a comparison of the morphological 

 value of these quartets of cells in the annelids, mollusks, and 

 platodes ; and the remarkable and interesting fact is now be- 

 coming apparent that while they do not have exactly the same 

 value in all the forms, they nevertheless show so close a cor- 

 respondence both in origin and in fate that it seems impossible 

 to explain the likeness save as a result of community of descent. 

 The very differences, as we shall see, give some of the most 

 interesting and convincing evidence of genetic affinity ; for 

 processes which in the lower forms play a leading role in the 

 development are in the higher forms so reduced as to be no 

 more than vestiges or reminiscences of what they once were, 

 and in some cases seem to have disappeared as completely as 

 the teeth of birds or the limbs of snakes. The processes in 

 question relate to the formation of the mesoblast in its relation 

 to the micromere-quartets, and on them the whole discussion 

 may be made to turn. 



The higher types — i.e., the annelids, gasteropods, and lamel- 

 libranchs — have for some time been known to agree closely in 

 the general value of the quartets. Rabl first demonstrated 

 that in Planorbis the entire ectoblast is formed from the first 



