118 DEVELOPMENT OF THE FROG'S EGG [Cn. XI 



watched during the later cleavage -period, it will be found 

 that the upper white surface disappears, and often a whitish 

 band is found in the position of the first furrow. Continuous 

 observation also shows that the white hemisphere may slowly 

 sink to one side. At thirty hours the blastopore has appeared 

 in the normal eggs, while on the inverted eggs two gastrula- 

 invaginations are found. From each half of the egg a more or 

 less complete embryo may develop (Fig. 36, B, C, D). The two 

 " double monsters " are united to each other in various ways, 

 often with the two ventral surfaces united in one common yolk- 

 mass, as shown in Fig. 36, B. Another of these double forms 

 is shown in Fig. 36, C, D, and a cross-section through the body 

 in Fig. 36, E. 



The details of these experiments of Schultze have not yet 

 been published. The method of gastrulation of the halves is 

 not clearly explained, nor does Schultze explain the changes 

 that take place in the interior of the blastomere after the 

 rotation. The results show, however, in the clearest way that 

 each half of the egg, after the first division, has the power to 

 develop all the organs of a single embryo. 



Wetzel ('95) has more recently studied the gastrulation-pro- 

 cess in some of these embryos and has given a fuller descrip- 

 tion than Schultze of the origin of the archenteron. A cross- 

 section through the blastula-stage of one of these eggs is shown 

 in Fig. 36, A. Two distinct segmentation-cavities are present 

 in the upper or white hemisphere of the egg. The centre of the 

 double blastula is filled with large yolk-cells. The sides are 

 formed of smaller cells richer in protoplasm and pigment. The 

 structure of this double blastula shows that, in all probability, 

 the contents of each of the first two blastomeres have rotated 

 after the inversion of the egg so that the more protoplasmic 

 portions have come to lie at the outer and upper sides of each 

 blastomere ; while the heavier yolk has sunken down to the 

 lower surface along the cell-wall that separated the first two 

 blastomeres from each other. 



At a later stage a depression appears on the surface of the 

 egg in the region of the plane that separated the first two blas- 

 tomeres from each other, i.e. approximately in the plane of the 



