Hyatt.] 1 26 [March 5. 



necessary to assume that unequal growth would always result in 

 invagination ; but it is easy to see that the more rapid growth of 

 one hemisphere, accompanied perhaps by an absorption of the 

 blastocoelic fluid, might lead to a double-walled gastrula." 



This hypothesis acquires support from the researches which 

 have been so far made into the direction of the planes of fission. 

 Dr. Arthur Kollmann in his treatise (Der Tastapparat der Hand, 

 etc., Leipzig, Voss, 1883) has announced an important law of 

 fission. Thus in the development of cells in the epidermis when 

 the predominating planes of fission are at right angles to the axis 

 and to the surface of the body, the epidermis increases in length ; 

 whereas, when the somewhat rarer form of cell division (in which 

 the planes of fission are at right angles to the surface, but par- 

 allel with the long axis of the body) occurs, the same membrane 

 increases in breadth ; and when the planes of fission are parallel 

 both to the surface and to the axis of the body the epidermis 

 increases in thickness. The same author also shows that the 

 epiblast in the neighborhood of the primitive streak consists 

 of elongated cells. These give rise to the primitive streak 

 below, and the cells change in form ; those above being subject 

 to considerable lateral pressure are elongated while those in 

 the primitive streak vary, being often cuboidal. Finally, how- 

 ever, these become similar to the forms from which they origi- 

 nated when in course of growth the same influence is again 

 brought to bear upon them. Dr. E. G. Gardiner, to whom we 

 owe the suggestion as to the correspondence of the modes 

 of division and forms of cells, and the direction of the pres- 

 sure (Beit. Kennt. d. Epitrich. Arch. Mikr. Anat., vol. xxiv, 

 1884) has shown that the cells of that part of the epiblast which 

 is destined to form the medullary canal of the embryo are at first 

 cylindrical cells, because of the lateral pressure of the epiblast ; 

 but towards peripheral parts where the activity of the fission, and 

 therefore pressure, is much less, the cylindrical cells are replaced 

 by cuboidal cells. At a later stage, after the medullary canal is 

 closed, the rapid development of this part exerts such a pressure 

 upon the superimposed epiderm that the cells become flattened 

 and form only a single layer. The same is true of the cells of the 

 epiderm immediately above the equally active protovertebrae 

 while between these places where growth and pressure are less 



