500 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxix. 



Fig. 78. Section froai a blastoderm like fig. 77, but with periblast deeply sunk in the 

 yolk, thus greatly enlarging the segmentation cavity. 



79. 1,024-blastomere stage, fourth type. A. ^^DcckschicJd" is forming and the 



periblast is giving rise to cells. 



Plate XI. 



80. Horizontal section tiirough a blastoderm of the same stage as fig. 78. Epi- 



dermic stratum very definite and in part two-layered, 



81. Late stage of segmentation. Section from a blastoderm intermediate 



between the gently arched and the thick-ended types. Blastomeres scat- 

 tered in the large segmentation cavity caused by the down-sunken periblast. 



82. Late segmentation stage, round-arched type. Epidermic layer present. 



Many cells resting on periblast and probably formed from it. The left 

 outer i>eriblast shows two multipolar spindles. 



83. Section through late lens-shaped blastula. The ''DeckscJiidd" is two-layered, 



and the periblast, which has no forming cells, is deeply sunken. 



84. Outline section of normal type late blastoderm. The cells have moved 



upward, forming a compact mass, the lower limit of which is marked x x, 

 and having a large subgerminal cavity. Giant nuclei in periblast. • 



85. Late blastoderm, second type, showing same structures as fig. 84. Periblast 



much sunken. 



86. Same stage, thick-ended type. Periblast is thin and multinucleate. A 



large binucleate cell is shown. 



87. Same stage and structures as above from a blastoderm like fig. 83. Many 



resting nuclei in periblast. 



88. Normal type blastoderm spreading over yolk preparatory to the beginning of 



invagination. 



89. Section from a ])lastoderm similar to the above save for a slight variation in 



shape. 



