LILLIE. [Vol. XVII. 



egg to a protozoan, and the cleavage stages to colonies of 

 protozoa. We are even beginning to doubt that pJiylogenetically 

 the protozoan-colony theory is a necessary view of the origin 

 of the metazoan body ; ontogenetically this view has become 

 impossible. 



But though the hypothesis I have just stated above appears 

 necessary to many at the present time, I think there are very 

 few who will follow Driesch in his rapid flight from one ex- 

 treme, the dominant cell theory which he upholds in his earlier 

 papers, to the opposite : that only incomprehensible and intangi- 

 ble vital forces, such as " Fernkrafte," are adequate to explain 

 the unity which, as we are beginning to see, pervades all the 

 metamorphoses of any species. I think that most naturalists 

 will resolutely suppress the feeling, almost of panic, that accom- 

 panies the revelation of the real complexity of the problems of 

 development, and will stand firm in the unassailable position 

 that, until it is shown that there enter into the composition 

 of protoplasm elementary substances not known elsewhere, 

 vital properties must be explained as in some way due to the 

 qualities of that matter which physicists and chemists investi- 

 gate. On the other hand, better understanding of the marvel- 

 ous properties of protoplasm furnish problems to the physicist 

 and must inevitably lead to a reconsideration of the properties 

 and theories of matter. 



Zur Strassen ('98) has been led by his studies on nematode 

 development to reject mechanical views of cleavage. He is 

 impressed by the adaptive nature of the earliest cleavages and 

 exclaims : " It is as though the cleavage cells possessed a perfect 

 guiding instinct r " Ich zogere wirklich kaum, diesen Begriff 

 alien Ernstes fiir das Verhalten der Blastomeren in Anspruch 

 zu nehmen." He gives a vivid description of the manner in 

 which the 7"-shaped early 4-cell stage is converted into a 

 rhomboid by active amoeboid migrations of the cells, some- 

 times carried out in the face of considerable difificulties, as in 

 the giant eggs, where the cells have to force past a constriction 

 of the egg-membrane. " Ich sah zwei Organismen ein Ziel, 

 ihre Vereinigung, erstreben und dieses Ziel erreichen. . . . 

 Die Zellen sind nicht Bausteine der Entwickelung, die durch 



