PART THIRD. 



THE MODE OF DEVELOPMENT OF ANIMALS CORRE- 

 SPONDS WITH THE TYPE OF THE GRAND DIVISION 

 TO WHICH EACH ONE SEVERALLY BELONGS. 



CHAPTER XVII. 



THE SEGMENTATION OF THE EGG. THE EMBRYOLOGY OF PROTOZOA, ZOOPHYTA, MOL- 

 LUSCA, ARTICULATA, AND VERTEBRATA. 



As a concluding lecture of this course, I propose to show that 

 the mode of development of an animal corresponds with the type 

 of the division to which it belongs. 



Of course it would not be possible in so brief a period to 

 present anything more than a sketch or outline of the processes 

 which are involved in the origination of the several parts of 

 the animal organism ; nor is it really necessary to go into the 

 minuter details in order to reach the characteristic features which 

 enable one to distinguish the several types from one another. 



I have already told you that the relation of the parts of an 

 organism constitutes its type. You have seen the relation to a 

 spiral in the Protozoa ; the repetition of parts across the body in 

 Zoo'phyta ; the uniformity of organization in the Mollusca ; the 

 repetition of parts from point to point along the axis of the body 

 in Articulata; and the body divided into an upper and lower 

 region by a vertebral column in the Vertebrata. These are 

 characters which are not hidden in the minute structure of each 

 particular organ, but lie on the surface as it were ; and this is 

 none the less true of the embryos of animals. 



I need but refer to the progressive steps in the formation of 

 the egg, to recall to your minds what I have said in regard to 



