180 THE DEVELOPMENT OF THE TADPOLE. 



apparently distinct sets of cells form but one series, having the 

 shape of a flask, the neck being the darker, and the body of the 

 flask the lighter portion. 



This flask-shaped cell is contained within a delicate sac, or 

 rather the contents of the cell appear darker than its surroundings. 

 From the great variations in form obtained from various sections 

 taken from animals of the same date, it becomes extremely 

 difficult to follow the real alterations in shape which this tube 

 undergoes for a day or two ; but from a careful examination of a 

 large number of sections of eggs taken from the 5th to the 7th 

 of March — that is, about a week after being deposited — it appears 

 as if the front portion of the original tube became bent at almost 

 a right angle and formed the fore and mid brain, whilst the longer 

 and thinner part became converted into the medulla oblongata 

 and spinal cord. 



The whole brain, together with the spinal cord, is by this time 

 perfectly distinct and well differentiated throughout its course 

 from the structures surrounding it, and has also undergone a great 

 change as to the form of its individual cells. This chiefly consists 

 in the great increase, both in number and size, of the dark portions 

 of the flask-shaped cells before mentioned, so that, except in the 

 thinnest sections, it is impossible to make out anything but a 

 black mass interspersed with lighter dots. The cerebrum has, 

 however, already commenced to become somewhat differentiated 

 into lobes, these taking the form of slightly darker aggregations 

 and swollen masses of cells, with lighter portions intervening. 

 The original hollow tube has in front become completely filled up, 

 and is only represented behind by the apparently double spinal 

 cord, which may, however, have a double appearance only from 

 being sections taken from a tube, and this, on a comparison of all 

 the sections from a single specimen, is found to be actually the 

 case, as the upper and lower sections of the cord show a single 

 line, whilst the division between the two portions of the cord is 

 greatest in the central section. 



It will be seen from an inspection of the diagram in Fig. 2, 

 Plate XVIII., of Part II., which gives a sketch of the animal at 

 this period, that it has no tail, a rudiment of this organ only 

 appearing; but in the course of three days the creature takes quite 



