No. 3.— CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY 

 OF THE MUSEUM OF COMPARATIVE ZOOLOGY AT HARVARD 

 COLLEGE, UNDER THE DIRECTION OF E. L. MARK, No. 202. 



Maturation, Fertilization, and Segmentation of Pennaria tiarella 

 {Ay res) and of Tubular ia crocea (Ag.). 



By George T. Hargitt. 



TABLE OF CONTENTS. 



Page 



I. Introduction 162 



II. Material and methods 163 



III. Observations 164 



A. Pennaria tiarella 164 



1. Growing eggs 164 



2. Formation of polar cells 166 



3. Fertilization 173 



4. Cleavage 179 



B. Tubularia crocea ... 179 



1. Oogonia 179 



2. Oocytes 180 



a. Nucleolus 183 



3. Polar-cell formation and fertilization 186 



4. Cleavage 188 



a. Early cleavage .... 189 



b. Later cleavage 190 



c. The formation of the germ layers 191 



d. Double nuclei 192 



IV. Discussion and historical review 194 



1. Growth of eggs 194 



2. Synapsis and reduction 195 



3. Polar-cell formation 197 



4. Nucleolus 198 



5. Cleavage 199 



6. Double nuclei 201 



V. Summary 203 



A. Pennaria tiarella 203 



1. Nucleolus 203 



2. Polar cells 203 



3. Fertilization 204 



4. Cleavage 204 



B. Tubularia crocea 205 



1. Oocytes 205 



a. Germinative vesicle 205 



b. Nucleolus 205 



