:.diTU. — Gastrulation in Aurelia. 



PLATE I. 



ABBREVIATIONS. 



bl'po. Blastopore. 



cau. sg. Segmentation cavity. 



cl. Immigrated cell. 



cedent. Coelenteron. 



cog. Coagulum. 



ec'drm. Ectoderm. 



en'drin. Entoderm. 



nl. Chromatic portion of degenerated nucleus. 



nl. ec'dnn. Nuclei of deeper portion of ectoderm. 



Figures 1-4. Sections to illustrate the nature of the invagination. 

 Fig. 1. An early stage of invagination, x 460. 

 " 2. A slightly later stage tlian that of Figure 1. X 540. 

 " 3. A stage in whicli the invagination is well advanced. X .385. 

 " 4. A gastrula with invagination completed. X 410. 

 " 5. Section of a gastrula cut in a plane (equator) perpendicular to the axis of 



the hlastoporic canal. X .385. 

 " 6. Section of an older individual through the equator, showing increase in 

 thickness of the entoderm, x 385. 



