38 DEVELOPMENT AND GROWTH OF 



to consist of a nucleus surrounded simply by granular proto- 

 plasm. 



At about this period of incubation the formative cells are 

 especially numerous at the periphery of the blastoderm, and, 

 no doubt, become converted into the mass of mesoblast which 

 is found at about the twenty-fifth hour in the region of the 

 vascular area. Some of them are lobate, and appear as if 

 they were undergoing division. At this time also the greatest 

 number of formative cells are to be found at the bottom of the 

 now large segmentation cavity. 



In embryos of from thirty to forty hours the cells of the 

 hypoblast have, over the central portion of the pellucid area, 

 entirely lost their highly refractive spherules, and in the fresh 

 state are composed of the most transparent protoplasm. When 

 treated with reagents they are found to contain an oval nucleus 

 with one or sometimes two nucleoli, imbedded in a considerable 

 mass of protoplasm. The protoplasm appears slightly granular 

 and generally contains one or two small vacuoles. I have already 

 spoken of the gradation of the hypoblast at the edge of the 

 blastoderm into white yolk. I have, therefore, only to mention 

 the variations in the size of its cells in different parts of the 

 pellucid area. The points where the cells are smallest seem 

 generally to coincide with the points of maximum growth. Over 

 the embryo the cells are more regular than elsewhere. They 

 are elongated and arranged transversely to the long axis of 

 the embryo. They are somewhat hexagonal in shape, and not 

 unlike the longer pieces in the dental plate of a Myliobatis 

 (PI. I, fig. 10). This regularity, however, is much more marked 

 in some specimens than in others. These cells are about ^J^yth 

 of an inch in breadth, and y^V^th in length. On each side of the 

 embryo immediately external to the proto-vertebrae the cells are 

 frequently about the same size as those over the embryo itself. 

 In the neck, however, and near the end of the sinus rhomboidalis, 

 they are considerably smaller, about -j^o^ 1 mc ^- eacn wa 7- The 

 reason of this small size is not very clear, but probably shews 

 that the greatest growth is taking place at these two points. 

 The cells, again, are very small at the head fold, but are very 

 much larger in front of this larger, in fact, than any other cells 

 of the hypoblast. Outside the embryo they gradually increase 



