148 



The invagination stage in marked by the presence at the surface 

 of a small circular pit. On one side of the pit, the anterior side, 

 and over a considerable surface of the egg, the cells multiply rapidly, 

 and thus mark off the embryonic area. The invaginate cells proliferate 

 and form a solid wedge-shaped mass which will be spoken of as the 

 »Keel« (Fig. 6). For fuller account see No. SO of Johns Hopkins 



Fig. <;. 



Fig. 6. Longitudinal median section through emhryonic area {EA), invaginate 

 cavity ( I) and cell mass of keel. The cells forming the keel are very small. They 

 are rapidly dividing, and are very intimately associated with spherules of yolk, as is 

 indicated in the drawing. 



DC = Traces of degenerating nuclei. 



Ep = superficial cell-layer ; the peripheral nuclei closely associated with a 

 peripheral tier of yolk spheres. 



Y = yolk. 



University Circulars). The pit, at first very slight becomes deeper 

 and broader, and is easily seen with the naked eye (Fig. 4 . Eventually 

 this depression entirely disappears and in its place, we have a plug or 

 core of cells extruding downward into the yolk, and backward in a 

 conspicuous train. 



By the time the naupliar appendages are outlined , the keel has 

 undergone remarkable changes , and is no longer visible as a promi- 

 nent object at the surface. Some of its cells pass forward under the 

 embryonic area; some take part in the thoracic-abdominal process, 

 while a very large number pass to various parts of the egg in a way 

 already described in an earlier paper already referred to. 



Some remarkable variations and irregularities occur in the keel 

 and egg-nauplius stages. There is frequently formed in connection 



