No. '2. — CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY 



OF THE MUSEUM OF COMPARATIVE ZOOLOGV AT HARVARD 



COLLEGE. E. L. MARK, Direltor. No. 132. 



JUl 



Oil' 



The Early Development of Lepas. A Study of Cell-Lineage 



and Germ-Layers. 



By Maurice A. Bigelow. 



TABLE OF CONTENTS. 



I. Introduction 



II. Historical 



III. Materials and methods . . 



I V. Maturation and fertilization . 



The unsegmented ovum. 

 Review of literature on 

 maturation and fertil- 

 ization 



V. General sketch of cleavage 



and germ-layers . . . 



VI. Nomenclature of cleavage . 



VII. Cleavage 



1. Introductory .... 



2. First cleavage. Two 



cells 



3. Review of the litera- 



ture on first cleavage 



4. Second cleavage. Four 



cells 



5. Review of the litera- 



ture on the second 

 and succeeding cleav- 

 ages ....... 



3. Third cleavage. Eight 

 cells 



7. Fourth cleavage. Six- 



teen cells . . . . 



8. Fifth cleavage. Thirty- 



two cells . . . . 



9. Sixth cleavage. Sixty- 



two cells. Closing 

 of the blastopore. 

 The germ-layers 

 10. Seventh cleavage. 

 The mesoblast . . 



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102 



104 



111 



PAQE 



11. Review of literature 



on late stagesof cleav- 

 age, on closing of 

 blastopore and on 

 differentiation of the 

 germ-layers . . . 113 



a. Late stages of 



cleavage . . . 113 



b. Closing of blasto- 



pore .... 114 



c. Differentiation of 



the germ-layers 114 



12. Determinate cleavage 116 



13. Notes on cleavage and 



germ-layers in L. 

 fascicularis . . . 117 

 VIII. Extension of the mesoblast 

 and entoblast. Later de- 

 velopment of the germ- 

 layers 119 



IX. Formation of the append- 

 ages of the Nauplius, and 

 development of the or- 

 gans 121 



X. General considerations on 



cleavage and cell-lineage. 122 

 XI. Comparison of the germ- 

 layers of Lepas with those 

 of other Crustacea . . 127 

 XII. General summary, with 

 table of cell-lineage of 



Lepas 133 



Addendum 136 



Bibliography 138 



Explanation of Plates 143 



