CHAS. B. WILSON. 



(d) Summary . . . . . 138 :. 



(e) Conclusions ..... 138 ; 



D. Segmentation: 



(1) Mode of cleavage ..... 141 



(2) Behaviour of polar bodies .... 141 



(3) Filose activities ..... 144 



(4) Contractile movements .... 145 



(5) Separation of blastomeres .... 149 



(6) Blastula 149 



(7) Summary . . . . . .149 



(8) Conclusions ...... 151 



E. Gastrulation : 



(1) Ciliation of the larva ..... 153 



(2) Formation of gastrula .... 154 



(3) The apical plate . . . . . 155 



(4) Change from radial to bilateral symmetry . . 156 



(5) Escape from egg-membrane .... 158 

 IV. The Pilidipm. 



A. Habits : 



(1) Locomotion . . . . . .159 



(2) Use of apical flagella . . . . .159 



(3) Food .161 



B. Morphology : 



(1) External morphology . . . . . 162 



(2) The primitive intestine .... 162 

 c. Mesoderm Formations : 



(1) Origin of mesenchyme cells .... 164 



(2) Their contractility . . . . .165 



(3) Development of the larval muscles : 



(a) Apical muscles .... 166 



(6) Interparietal muscles . . . . 167 



(c) (Esophageal muscles .... 168 

 {d) Circumoral muscles .... 169 

 {e) Parietal muscles .... 170 

 (/) Lappet muscles .... 171 



(4) Summary ...... 174 



(5) Conclusions ...... 175 



D. Histology: 



(1) The epidermis . . . . .181 



(2) The apical plate . . . . .182 



(3) The cilia rows . . . . .185 



(4) The intestinal canal ..... 188 



(5) Summary 189 



