CONK LIN. [Vol. X 1 1 1 . 



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II. The Segregation of the Ectoblast 54 



1. Formation of the First Quartette of Micromeres 54 



2. Formation of the Second Quartette of Micromeres 57 



3. Division of the First Quartette of Micromeres and Formation of 



the Turret Cells (Trochoblasts) 58 



4. Formation of the Third and Last Quartette of Micromeres and 



Complete Segregation of the Ectoblast 60 



5. Division of the Second Quartette of Micromeres 63 



III. The Segregation of the Mesoblast and Entoblast 67 



1. Formation of the Mesentoblast, the First Member of the Fourth 



Quartette 67 



2. The Primary Enteroblasts 68 



3. The Primary Mesoblasts 69 



4. Completion of the Fourth Quartette and Rotation of Ectoblast.. 75 



5. The Four Macromeres, or Basal Quartette 80 



IV. History of the First Quartette of Ectomeres 82 



1. The Ectoblastic Cross 83 



a. Its Formation 83 



b. Axial Relations - 84 



c. Later History 85 



d. Significance 91 



2. The Turret Cells 106 



3. Organs formed from the First Quartette 109 



a. Head Vesicle 109 



b. Apical Sense Organ 109 



c. Cerebral Ganglia and Eyes iio 



d. Cerebral Commissure in 



e. Cerebro-pedal Connectives in 



/. Apical Cell Plate 112 



g. Preoral Velum (in part) 112 



V. History of the Second and Third Quartettes of Ecto- 



meres 113 



1. The Second Quartette. Comparisons 114 



2. The Third Quartette. Comparisons 122 



3. Organs formed from the Second and Third Quartettes 128 



a. Blastopore, Stomodaeum, and Mouth 129 



b. Posterior Growing-Point 130 



c. Velum 132 



d. Shell Gland 140 



e. Foot 141 



/. External Excretory Cells 143 



VI. History of the Mesomeres 148 



1. The Mesoblastic Bands 148 



2. The Scattered Mesoblast Cells. Comparisons 149 



VII. History of the Entomeres 153 



1. The Four Macromeres 153 



2. The Fifth Quartette 154 



3. The Fourth Quartette 156 



4. The Enteroblasts 156 



5. Organs formed from the Entomeres 157 



