176 AETHUE E, SHIPLEY, 



equatorial, but nearer tlie upper than the lower pole. After 

 its appearance the epiblastic half is separated from the hypo- 

 blastic or yolk-bearing half (fig. 2). 



The external phenomena of segmentation have been accu- 

 rately described by Max Schultze, with the exception of 

 the next stage. After the first equatorial furrow he describes 

 two more in the same plane, but in my eggs the equatorial 

 furrow was followed by two vertical lines, which appear at 

 first in the upper pole exactly as they do in the Frog's ovum 

 (fig. 3). These are followed by two more equatorial furrows 

 which divide the egg into thirty-two segments. After this the 

 segments of the epiblastic pole divide more rapidly than those 

 of the lower. 



Fig. 5 represents a transverse section through an egg thirty- 

 six hours after fertilisation. In this stage it is a blasto- 

 sphere, with a segmentation cavity enclosed by a single layer 

 of cells except along the line where the epiblastic and hypo- 

 blastic cells join. Here the layer is two cells thick. The 

 nuclei of the large cells appear small, but it must be recollected 

 that the amount of protoplasm is very small compared to the 

 yolk. The latter has been omitted for the sake of clearness. 

 Fig. 6 is taken from an egg twelve hours later. Here both 

 the roof and floor of the segmentation cavity are many cells 

 thick. A similar stage is found in the Frog's ovum, but there 

 is this difi'erence between the two. In the Frog's egg the 

 whole of the roof of the segmentation cavity forms epiblast ; 

 in the Lamprey it is only the outermost layer. The following 

 stages are accompanied by a thinning out of the roof of the 

 segmentation cavity, and are represented in figs. 7 and 8. 

 On this point my observations tend to confirm those of 

 Calberla, and are opposed to those of Schultze, who found 

 a many layered roof to the segmentation cavity just before 

 invagination. The thinning out appears to be brought about 

 by the inner cells of the roof passing round to the sides and 

 floor of the segmentation cavity. Just before the invagination 

 which forms the gastrula the roof of the segmentation cavity 

 consists of a single layer of cells ; the segmentation cavity is 



