No. 7. — Tlie Early Embryology of Ciona intestinalis, Flem- 

 ming (L.). 1 By W. E. Castle. 



CONTENTS. 



Page 



I. Introduction 203 



II. Material, Life History . . . 205 



III. Methods 213 



1. Killing, Preservation . . 213 



2. Decortication, Staining, 



Mounting 214 



IV. Maturation and Fertilization . 217 

 Summary on Maturation and 



Fertilization 223 



V. Polarity of the Egg .... 224 

 Summary on Polarity of the 



Egg 226 



VI. Cell Lineage of the Embryo . 226 



1. Nomenclature 226 



2. Cleavage 227 



A. Early Stages of Cleav- 



age 227 



(a) To 24-cell Stage . . 227 



(b) Summary on Early 



Cleavage Stages . 234 



B. Later Stages of Cleav- 



age 235 



(a) From 24-cell to 46- 



cell Stage . . . 235 



(b) 48-cell Stage . . . 237 



(c) 64-cell Stage ... 239 



(d) 76-cell Stage ... 240 



(e) Summary on Later 



Cleavage Stages . 242 



3. Gastrulation 243 



Page 



A. Early Stages of Gastru- 



lation 243 



(a) 112-cell Stage . . 243 

 (6) Differentiation of 

 the Principal Or- 

 gans as seen at 

 the 112-cell Stage 245 

 a. Topographical . 245 

 $. Histological . 247 



B. Later Stages of Gastru- 



lation 249 



(a) From 112-cell to 



128-cell Stage . 249 

 (6) Closure of the Blas- 

 topore .... 252 



C. Summary on Gastrula- 



tion 261 



4. Formation of the Larva . 263 

 Summary on Formation 

 of the Larva .... 265 

 VII. Discussion of some Theoreti- 

 cal Questions 267 



A. Origin of the Germ Layers 



of Chordates .... 267 



B. The Coelom Theory . . 271 



C. Ancestry of the Chordates 272 

 VIII. Conclusions 274 



IX. Table of Cell Lineage . . . 275 



Literature Cited 276 



Explanation of Plates 280 



I. INTRODUCTION. 



So long ago as 18GG, Kowalevsky wrote, " Die Entwirklune;sgeschichte 

 der Ascidieu wurde schon vielfach studirt." If this statement was true 



1 Contributions from the Zoological Laboratory of the Museum of Comparative 

 Zoology at Harvard College, under the direction of E. L. Mark, No. LII. 

 vol. xxvii. — no. 7. 1 



