OF THE TADPOLE. 37 



about one day old — is seen, in Plate I., Fig. 2, a and b, to consist 

 of an undifferentiated yelk mass forming the body of the egg, 

 surrounded by a dark layer of cells, the epiblast, under which we 

 have a layer of larger cells, called the hypoblast. From the epi- 

 blast it will be seen in future plates that the epidermis, the 

 nervous system, and most of the organs of special sense, are 

 produced, whilst the hypoblast forms the whole of the epithelial 

 lining of the alimentary canal, with its glands, excepting the lips 

 and anus, which, like the external skin, are formed from epiblast. 

 These two layers are well seen in Plate I., Fig. 3, where the epi- 

 blast is seen as a dark line of cells, the hypoblast consisting of a 

 looser texture below. At the light pole of this figure may be 

 seen a row of nucleated cells, which are well defined and marked 

 off from the great body of non-nucleated cells, at this time form- 

 ing the bulk of the yelk mass. It is from the different growth ot 

 the nucleus in the cells that the distinctive character of each kind 

 of tissue may be traced. 



The epiblast soon spreads over the whole surface, with the 

 exception of a small hole called the blastopore, through which an 

 invagination of the epiblast occurs, and after this has gone on for 

 some time, a flattening of the dark pole takes place (Plate I., Figs. 

 4, 5, and 6), and a delicate layer of cells, called the mesoblast 

 (Plate II., Figs, i, 2, 3, 4, etc.) forms between the epiblast and 

 hypoblast. This mesoblast, although barely visible at first, is yet 

 an extremely important part of the structure, as in the course of 

 development it forms the bones, muscles, connective tissues, and 

 blood-vessels. 



The flattening of the dark pole was first observed on March 

 2nd, the third or fourth day after deposition. This flat spot soon 

 deepened down into a curve, the outer rim at the same time rising 

 up, forming a decided ridge or cornua, the sides of which, 

 gradually approaching each other, produce a medullary groove, 

 below which the most interesting and extensive modifications 

 and re-arrangement of the three layers of hypoblast, mesoblast, 

 and epiblast are occurring. These will be treated in a future 

 paper. 



