SEGMENTATION OP THE OVUM OF THE SHEEP. 239 



difference between outer and inner cells dates from the first 

 division of the ovum. 



(2) Keibel (38) states that he has been able to observe in a 

 number of cases in the rabbit the blastopore of van Beneden. 



(3) Heape (29) has seen a similar appearance in the seg- 

 mented ovum of the mole. 



(4) Duval has described the formation of a metagastrula as 

 a result of observations by sections in the case of the bat's egg. 



(5) In the sheep, also, 1 have found a stage exactly com- 

 parable to the metagastrula, though I place a different inter- 

 pretation upon it. 



On the other hand, the value of van Beneden's and Heape's 

 observations must be to a certain degree discounted, because 

 they were based upon optical sections only. 



Keibel does not state whether his observations are from real 

 or optical sections. 



Then, again, although Heape finds a metagastrula stage at 

 the end of segmentation, he does not confirm van Beneden's 

 account of its formation. On the contrary, he considers that 

 it is formed by the migration of yolk granules from the outer- 

 most segments to the innermost at the close of segmentation. 



He writes (p. 168) : '' I have myself examined segmenting 

 ova of the rabbit, and have isolated the segments one from the 

 other, in order the more clearly to compare them, and in no 

 case have I been able to distinguish the slightest difi'erence in 

 the density or constitution of these segments. If my observa- 

 tions are correct, then the differentiation of the segmentation 

 spheres into two layers in the fully segmented ovum is not 

 a primary differentiation, such as Beneden discerns, but a 

 secondary differentiation, due to the peculiar circumstances of 

 nutrition and development attending the formation of the 

 Mammalian embryo." 



According to Heape, it is only at the close of segmentation 

 that the segments become divided into two kinds. " A single 

 layer of cubical hyaline segments completely surrounds, except 

 at one point, an inner mass of rounded or polygonal densely 

 granular segments." Thus Heape will not admit that there is 



