The Development of Ischnochiton. 633 



expression of the action of intrinsic forces which control the de- 

 velopment from the earliest stages on to the end. Mechanical forces 

 and conditions such as gravity, surface tension, cohesion and pressure 

 undoubtedly are operative, but they are not believed to be the con- 

 trolling, coordinating agents in development. The early cleavages 

 accordingly are as important as those occurring in later life, and may 

 even be considered more so (cf. Watasé, '90). Also the long con- 

 tinued resemblances which exist in the development of flatworms. 

 Annelids and Molluscs, from the earliest segmentation of the egg, are 

 as fundamental and deep seated as are the homologies existing in 

 the adults. 



The list of these resemblances in Annelids and Molluscs is con- 

 stantly increasing. In all forms accurately studied the first three 

 quartettes of cells constitute the sum total of the ectoblast ; the meso- 

 blast arises at the fourth division of the posterior macromere; the 

 remaining members of this quartette and the macromeres become 

 entoblast: while the divisions and positions of the cells up as far as 

 the 28 cell stage are identical. Beyond this point Wilson ('92) 

 believed a divergence between the two classes ensued, and that the 

 development proceeded upon two entirely different lines. However 

 subsequent observations have not confirmed this belief, but have rather 

 served to show that the supposed differences were superficial or non- 

 existent and therefore the points of resemblance became more numerous 

 and extended throughout a longer period of development. 



The above characters have been shown by Lillie ('95) to exist 

 in the Lamellibranchs, and also in both classes there is an essential 

 similarity between the development of the first somatoblast. In An- 

 nelids this structure develops to a greater extent than in Unio but 

 the two have many points in common. 



Mead ('97) and Conklin ('97) in studies appearing about the 

 same time recognized the above resemblances and presented several 

 other important points. The rosette series was shown to have the 

 same origin and position in both Annelids and Molluscs, and it was 

 considered in the highest degree probable that in both it gives rise 

 to the apical sense organ. Conklin held that it also gives rise to 

 the cerebral ganglia while Mead considered it as yet an open question. 

 Mead further showed that the primary and secondary trochoblasts 

 have identically the same fate in at least two Annelids; the "head 

 kidneys" in AmpJiitrife develop from the same cells as in Nereis; 

 and the entoderm arises from the same cells in the 5 Annelids he 



41* 



