Nachdi-uck verboten. 

 Uehersetzungsrecht vorbehalten. 



The Development of Ischnochiton. 



By 

 Harold Heath, 



Assistant Professor of Zoology at Leland Stanford Jr. University. 

 With Plates 31—35 and 5 Figures in text. 



I 



Contents. 



Introduction. 



Ischnochiton magdalenensis. 



1. Natural History. 



2. Breeding Habits. 



3. Methods. 



4. General Sketch of Development. 



5. Nomenclature. 



I. The Unsegmented Ovum. 

 II. The Primary Cleavages. 



1. The Pirst Cleavage. 



2. The Second Cleavage. 



III. The Formation of the Ectoblast. 



1. Pirst Quartette of Ectomeres, 



2. Formation of Primary Trochoblasts, and the Second Quartette 

 of Ectomeres. 



3. Formation of Apicals, and the Third and Last Quartette of 

 Ectomeres. 



4. Division of Primary Trochoblasts; Formation of Accessory 

 Primary Trochoblasts; First Cleavage of Second Quartette. 



5. Formation of Tertiary Stomatoblasts ; Cleavage of Apicals ; 

 Origin of Secondary Trochoblasts (Tip Cells) ; Division of 

 Basais ; Second Cleavage of Primary Trochoblasts ; Formation 

 of Left Post-Trochal Cells. 



6. Origin of Posterior Secondary Stomatoblast. 



7. Second Cleavage of Third Quartette; Division of Accessory 

 Trochoblasts. 



IV. The Formation of the Mesoblast and Entoblast. 

 1. Primary Mesoblast. 



Zool. Jahrb. XU. Abth. f. Morph. g^ 



