THE DEVELOPMENT OF THE TADPOLE. 



a new form, possesses a well-developed tail, has acquired swimming 

 powers, and with them the brain has undergone a complete change 

 in form, the whole length from the top of the tail to the very apex 

 of the head becoming once more divided into two parts. Meeting 

 at the upper portion, these parts diverge most widely in the head 

 across the line of the auditory capsules, and these once again take 

 the form of a somewhat elongated tube compressed in the centre, 

 shown in Fig. 4, Plate XX., Vol. I., N.S. Although there is a 

 great diversity of form in the shape of individual cells, yet a great 

 majority of them are still flask-shaped, but with immensely 

 attenuated, jet-black necks, giving especially to the medulla oblon- 

 gata the appearance of transverse striation, as the vast majority 

 of the cells all lie parallel to each other across the axis of the 

 cord. 



The next noticeable alteration in form is occasioned by a 

 transverse division across the middle line of the brain, thus 

 distinctly making a separation between the fore and mid brain, 

 which had only previously been marked by the general configura- 

 tion. At this period also both the eye and ear sacs are partially 

 enclosed by cells which have still the same character as those of 

 the brain proper, but from the portion of the brain which 

 ultimately forms the optic lobes, several sections of March 19th 

 show a chain of oval bodies running through the general mass 

 from the front part of the immature optic lobes to the back of 

 the eye. The links, so to speak, of the chain are not more than 

 one-thousandth of an inch in length and about one-half this in 

 breadth, and probably represent an early stage in the formation of 

 the optic nerve. The changes occurring at this time also include 

 the formation of the ventricles by the approximation of the sides 

 of the double brain in some places and their separation 

 immediately below, thus forming irregular triangular spaces, the 

 central one, however, consisting of two triangles placed base to 

 base. These spaces or ventricles are not yet lined by any mem- 

 branes, but the necks of the flask-shaped cells previously mentioned 

 may be observed spreading out over the surface and uniting so as 

 to form a layer which soon becomes distinctly different from the 

 remaining portion, in forming a chain of cells arranged end to 

 end, instead of being crowded into parallel masses. This 



