368 WILSON. [Vol. VI. 



field in the study of annelid embryology, but, as I believe, 

 formed a new point of departure for a re-examination of the 

 entire germ-layer theory. Whitman showed (in Clepsine) that 

 the entire germ-bands of the trunk could be traced back to five 

 pairs of cells (teloblasts), whose origin in the cleavage was 

 accurately determined. One pair (primary mesoblasts) gave 

 rise to the mesoblast-bands ; one pair (neuroblasts) to the ven- 

 tral nerve-cord; two pairs (" nephroblasts ") to the trunk-ne- 

 phridia, and one pair (lateral teloblasts), as he conjectured, 

 were perhaps concerned in the origin of the muscles. That 

 an entire system of organs, such as the ventral nerve-cord, or 

 the trunk-nephridia could be traced back to a single blastomere 

 was a fact so extraordinary that many morphologists, Balfour 

 among them, at first refused to credit Whitman's statements, 

 notwithstanding the fact that the origin of the entire mesoblast 

 from a single cell had been established in a number of cases. 

 Later investigation, however, not only confirmed Whitman's 

 discoveries, but extended them to other Hirudinea and to the 

 Oligochasta ; and the fact that the special interpretation placed 

 by him upon the "nephroblasts" has been disputed does not 

 lessen the importance and significance of the work. Whitman's 

 researches showed that the material for complicated adult organs 

 might be so condensed and accelerated in development as to be 

 set apart by a single stroke, as it were, in the early stages of 

 cleavage, long before the establishment of the gastrula ; and this 

 fact opens up a long vista of possibilities regarding the second- 

 ary modification of the gastrula stage. It may be urged that 

 these modifications can have little general interest for the very 

 reason that they are secondary and take place only in a highly 

 modified type of development. The reply to this is, how do we 

 know what is the primitive type of gastrulation t The present 

 state of embryology certainly does not enable us to give any 

 positive answer to this question. Whether the primary form 

 is the epibolic or the embolic gastrula, the plakula, the unipolar 

 or multipolar delaminate planula, or a still different type, re- 

 mains to be seen ; and the very fact that the differentiation of 

 the layers is effected in such a diversity of ways proves conclu- 

 sively that these early stages of development are as susceptible 

 to secondary modification as the later. I shall show, further on, 

 that the history of the mesoblast in Nereis shows how a slight 



