42 BIOLOGICAL LECTURES. 



from the fact that the earlier cleavages are more constant than 

 the later ones. In all gasteropods, lamellibranchs, and anne- 

 lids, so far as known, the early cleavages are almost identically 

 the same; but in later stages there are certain differences in 

 the cleavage of various species and genera, many additional 

 cells, for example, being found in large eggs which are not 

 found in small ones. Thus, whatever the size of the ^^^, three 

 and only three quartettes of ectomeres are formed, which in all 

 cases occupy relatively the same positions and give rise to the 

 same organs. This is a fact of the widest application and of 

 the highest significance; it occurs in equal and unequal cleav- 

 age and in eggs varying in size from a few microns to more 

 than a millimeter in diameter. However, in the subdivisions 

 of these quartettes marked differences sooner or later appear. 

 In Crepidula plana, fornicata, convexa, and adunca the relative 

 volumes of the eggs are as i, 2|, 8|, 27|, and yet up to the 

 52-cell stage there is not a single difference in the cleavage of 

 these four species; but at this stage a single additional ecto- 

 derm cell appears in the large Q.gg of C. adunca, due to the 

 additional subdivision of one of the ectomeres; at the 82-cell 

 stage there are three additional ectomeres ; at a similar stage 

 all the other species have the same number of cells, that is, three 

 less than adunca, but in later stages the ectoderm cells divide 

 more rapidly in all the large eggs than in the small ones, for at 

 the time of the closure of the blastopore the number of ecto- 

 derm cells in the four species, plana, fornicata, convexa, and 

 adunca, are in the following ratio: i, 1.6, 2.6, 5. Finally, in 

 the adult condition these proportions are reversed, the largest 

 ^00 giving rise to the smallest individual with the smallest 

 number of cells. 



This difference in the number of cells offers no difficulty to 

 the doctrine of cell homology unless we assume that all divi- 

 sions are differential, a thing which we know is not true. After 

 blocking out the protoblasts of various regions and organs an 

 indefinite number of non-differential divisions may occur either 

 before or after the complete differentiation of the parts, and 

 this probably explains the larger number of cells in the embryo 

 of C. adunca and the smaller number in the adult. In fact. 



