No. 2. CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY 



OF THE MUSEUM OF COMPARATIVE ZOOLOGY AT HARVARD 



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



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

 and Germ-Layers. 



By MAURICE A. BIGELOW. 



TABLE OF CONTENTS. 



PAGE 



I. Introduction 62 



II. Historical 63 



III. Materials and methods . . 64 



I V. Maturation and fertilization. 



The unsegmented ovum. 68 

 Review of literature on 

 maturation and fertil- 

 ization 71 



V. General sketch of cleavage 



and germ-layers ... 73 



VI. Nomenclature of cleavage . 74 



VII. Cleavage 77 



1. Introductory .... 77 



2. First cleavage. Two 



cells 77 



3. Review of the litera- 



ture on first cleavage 85 



4. Second cleavage. Four 



cells 89 



5. Review of the litera- 



ture on the second 

 and succeeding cleav- 

 ages 91 



8. Third cleavage. Eight 



cells 98 



7. Fourth cleavage. Six- 



teen cells .... 100 



8. Fifth cleavage. Thirty- 



two cells .... 102 



9. Sixth cleavage. Sixty- 



two cells. Closing 

 of the blastopore. 

 The germ-layers . 104 

 10. Seventh cleavage. 



The mesoblast . . Ill 



VOL. XL. NO. 2 2 



11. Review of literature 



on late stages of cleav- 

 age, on closing of 

 blastopore and on 

 differentiation of the 

 germ-layers . . . 113 

 a. Late stages of 



cleavage . . . 113 

 6. 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 

 XL 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 



