No. I.] THE EMBRYOLOGY OF CREPIDULA. 6 1 



twelve cells (increased to sixteen by the division of the first 

 quartette) comes the whole outer covering of the body, the 

 shell gland and ciliated locomotor apparatus, the larval excre- 

 tory cells and stomodaeum, the nervous system and sense 

 organs. 



The material of the macromeres is not homogeneous as yet, 

 since one of them, the left posterior, contains most of the 

 future mesoblast ; but at this early stage we have two layers, 

 ectoblast and mesentoblast, perfectly differentiated. 



It is a most remarkable fact that in all annelid and molluscan 

 eggs with holoblastic segmentation the ectoblast is segregated 

 in just three groups or quartettes of cells — no more and no 

 less. The evidence for this remarkable fact has been accumu- 

 lating until at present it is known to be true of at least a score 

 of forms, and not a single trustworthy observation can be urged 

 against it. A few apparent exceptions have been recorded 

 among prosobranchiate gasteropods. Bobretzky ('77), after 

 describing the formation of the first two groups of micromeres 

 in Nassa, says : " The large spheres continue to bud off new 

 cells around the circumference of those already formed, while 

 the latter continue to divide." Of Fusus he says that after 

 the first two cleavages the segmentation goes on as in Nassa. 

 Of these statements it need only be said that the work was 

 done at a time (1874-75) when little attention was paid to the 

 details of cleavage, and confessedly the number of quartettes 

 of ectomeres was not known. I have worked over the cleavage 

 of Illyonassa and Urosalpinx, which are the nearest representa- 

 tives of Nassa accessible to me, and, although the form of 

 cleavage is very similar to that given for Nassa, there is no 

 departure from the rule that three, and only three, quartettes 

 of ectomeres are formed. 



Another exception is recorded by McMurrich ('86) for Fulgur. 

 After describing the formation of the first two groups of micro- 

 meres, he says : "The succeeding stages of segmentation I did 

 not follow in detail, but can state that they result in the increase 

 of the number of micromeres, partly by the division of those 

 already formed, and partly by the separation of new ones from 

 the macromeres. ... It would seem that in Fulgur spherules 



