The Microscope. 



81 



Griffith, and I will leave it to their judgment whether I or my 

 critics have written in the spirit of controversy. 



Parsons College, Fairfield, Iowa. 



OUTLINES OF EMBRYOLOGY. 



THIRD PAPER. 



cL 



Invagination and Formation of Germ Layers. 



THE process of gastrulation described in the last number only 

 occurs in eggs with a regular segmentation. When the cells 

 are unequal a modification ensues which is sometimes carried 

 so far that the hypoblast consists of but a few, or even one cell, 

 over which the epiblast spreads itself, as shown in the accom- 

 panying figure. In this case the archenteron is absent, and the 

 blastopore is represented by the edges of the epiblast. 



Pig. 1. Diagram of first and second segmentation of egg; at a, a nucleus is 

 in process of division, showing amphiaster or " spindle kern." 



Pig. 2. Segmenting egg of mollusc (Tergipes); the larger cells are macro-" 

 meres, the smaller, micromeres. 



Fig. 3. Diagrammatic section of blustula: b, blastoderm; s c, segmentation 

 cavity. 



Connected with the closure of the blastopore are many im- 

 portant things to be considered. It has long been known that 

 the hypoblast forms the lining of the middle region of the ali- 

 mentary tract, and hence the supposition was a natural one that 

 this opening which connected this portion with the exterior 

 should be either the mouth or the vent of the adult, but which 

 one, was uncertain. In some it persisted as one, in other em- 

 bryos as the other, and in still others it closed completely. Was 

 the blastopore a different structure in all these various forms ? 

 It was not until Balfour studied the development of a curious 

 worm-like animal known as Peripatus, that these various views 



