BY DR. KLEIN. 163 



two lower, are rounded off at their inner angles, i. e., those 

 turned towards the centre of the germ, as if the}' toad retracted 

 from it ; the lower two, also, are somewhat rounded at their 

 inner angles, but not so markedly as those above: by this 

 means a small cavity is formed, which lies just in the place 

 where the four segments meet. In sections of progressively 

 later stages, it will be observed, in the first place, that the 

 upper segments have undergone cleavage much more rapidly 

 in other words, that their elements are considerably smaller ; 

 and, secondly, that the cavity becomes enlarged at the expense 

 of the upper half of the germ. In a still later stage of cleavage, 

 forms will be met with in which the cavity takes up the greater 

 part of the space occupied by the upper segments. The cavity 

 is spanned by a thin dome, consisting of only two or three 

 layers of small elements; whilst its floor is flat and lined by 

 larger elements belonging to the lower segments. Under- 

 neath these elements, which still contain pigment, elements 

 occur which become larger as the white pole is approached. 

 At this time it may be observed, that these large elements 

 which may be termed u formative elements" spread upwards 

 from the floor of the cavity over the under surface of the dome, 

 until at last a stage is reached at which the whole of that sur- 

 face is covered with them. In the middle part of the dome 

 these formative elements are disposed in a single layer ; on 

 the parts which are in closer proximity to the floor of the 

 cavity, the number of layers is greater. The dome consists, 

 therefore, at this stage, in the first place, of two or, at most, 

 three layers of small elements which originally belonged to it 

 (and which are also continuous with the cortex of the rest of 

 the germ) ; and second!}', below these, in its central part, of a 

 layer of larger elements, which before formed part of the floor 

 of the cavity. 



Simultaneously with the changes just mentioned, another 

 important change occurs at the white pole, as may be ascer- 

 tained by the study of sections at different stages. This pole 

 has been getting gradually smaller, and now presents the 

 appearance of a sharply bounded white patch of the size of a 

 pin's head the so-called yolk-plug (Dotterpfropf). A fissure 

 occurs, which constantly extends further and further upwards, 

 increasing at the same time in width, until it gradually ex- 

 pands to a cavity, which is eventually only separated from the 

 cleavage-cavity by a single layer of the larger elements. As 

 this cavity (called the visceral cavity, Rusconi's cavity, 

 Leibeshbhle) increases, the cleavage-cavity diminishes. In 

 consequence of these changes, the position of the egg is 

 altered ; that which before was the upper half now becoming 

 the lower. (As regards the formation of the cleavage and 

 visceral-cavities, compare figs. 169-173.) 



