28 BIOLOGICAL LECTURES. 



exceptions being some of the earthworms and leeches referred 

 to above, in which the typical relations seem to have been dis- 

 turbed through changes in the nutrition of the embryo. Wher- 

 ever the typical quartet formation takes place — and this is the 

 case in nearly all the forms that have been adequately examined 

 — the general value of the quartets is the same, the first three 

 giving rise to the entire ectoblast, the fourth giving rise, one 

 cell to the mesoblast-bands and the other three to entoblast, 

 while the remnants of the primary quadrants, including the fifth 

 quartet if one is formed, give rise to the entoblast. This result 

 seems almost too simple and produces an impression of artifi- 

 ciality which may probably account for the reluctance with which 

 it has been accepted in some quarters ; but I think it is not too 

 much to say that few facts in embryology have been more 

 patiently studied or more accurately determined. The above 

 statement does not, however, contain the whole truth ; but 

 before completing it we may advantageously turn to the 

 development of the Tiirbellaria. 



It was long since shown by researches, beginning with Hallez 

 and Gotte and culminating in those of Lang, that the cleavage 

 of polyclades shows an extraordinarily precise resemblance to 

 that of the annelids and mollusks. Taking Lang's work on 

 Discocoelis as a type, we find four quartets of cells successively 

 produced from the primary or basal quadrants, following exactly 

 the same law of displacement as in the higher types, assuming 

 the same arrangement, and in their subsequent subdivision up 

 to a relatively late stage following so exactly the plan of the 

 annelid ^gg that even a skilled observer might easily mistake 

 one for the other (Fig. 4, A). Despite this accurate agreement 

 in the form of cleavage, Lang's observations seemed to show 

 that the cell-quartets had a totally different value from those 

 of the higher forms ; for he believed the first quartet to pro- 

 duce the entire ectoblast, the second and third to give rise to 

 the mesoblast, while the fourth quartet, with the basal cells, 

 formed the entoblast (Fig. 2, B). Such a result was more than 

 a stumbling-block in the way of the comparison. It was sub- 

 versive of the whole cell-lineage program ; for it seemed to 

 show that the cell-lineage of derivative animals {i.e., annelids 



