254 Edmund B. Jf'Uson. 



(prov^ed by direct observation in Clepsine, Rhynchelmis, Myzos- 

 toma and DentaUiim) . In the medusae it appears from the work, 

 of Zoja ('95) and Maas ('01), that the primary segregation is 

 not visibly polarized, but concentric; and qualitative division (de- 

 lamlnation) does not take place before the fifth cleavage.^ It 

 seems evident that in these differences of form and degree of the 

 initial segregation pattern we find the leading principle for an 

 explanation of the differences in mode of development shown by 

 isolated blastomeres of the various forms ; though as pointed out 

 beyond, a complete explanation Is not given by these facts alone. 

 To consider one or two more detailed Instances, the first division 

 of the first quartet cells in Patella Is qualitative, not merely In a 

 descriptive or prospective sense, but actually, as Is proved by ex- 

 periment. By the same standard, the second division of these 

 cells Is qualitative In the upper cell (i^), but only quantitative 

 in the lower one (i"). Such facts as these give the strongest 

 ground for the conclusion that all the divisions that would be 

 considered as qualitative or quantitative from thfc point of view 

 of descriptive cell-lineage, are really such as regards the inherent 

 factors of differentiation. The descriptive and comparative study 

 of cell-lineage represents something more, therefore, than a mere 

 enumeration of successive cell divisions and their geometrical re- 

 lations, and has the value of a direct examination of the normal 

 morphogenic process. 



These conclusions may appear to conflict with the view that 

 has been frequently urged by embryologists In late years that the 

 organism develops essentially as a whole, and that cell-formation 

 plays but a subordinate part in the morphogenic process. The 

 conflict is, I believe, only a seeming one. Roux has repeatedly 

 pointed out that the mosaic-principle Is by no means Irreconcll- 



'^Cf. Maas: "Wenn die (cytoplasmic) Substanzen in alien Radien, resp. 

 Axen, gleichmassig verteilt sind, wie bei den kugeligen Eiern von Medusen. 

 dann und nur dann hat man ein wirklich isotropcs Ei; in anderen Fallen, wo ein 

 polare Anordnung festgestellt werden kann, wie bei den echinodermen. bes- 

 teht die Isotropic niir urn eine hcstimmtc Axe; in weiteren Fallen kommt durch 

 Gestalt des Eies, wie bei den Cephalopoden, oder durch Lagerung der Substan- 

 zen wie bei den Ampbibien, eine bilateral-symmetrisch Anordnung zu Stande 

 und in anderen Fallen ist diese Anordnung noch etwas komplizierter (siehe 

 Z. B. Mycosotoina)." ('03, p. 72.) 



