M. Ussow's Zoolo(jico-Emhryological Investigations. 107 



opposite the naiTOW segments just mentioned, two very lai<'e 

 segmentation-spheres are now separated by eonstrictiim 

 (dm-ing tlie fourth hour) ; and tliese phice themselves direetly 

 opposite to the two primitive s})liercs. In this way, in about 

 lour hours from the commencement of segmentation, there are 

 produced eight segments and four spheres of segmentation. 

 From these four, and ten subsequently produced spheres of 

 segmentation, originates, by means of further spontaneous 

 division (longitudinal division), tjie central part of the ger- 

 minal disk. 



In the subsequent stages we observe the following : — 1, a 

 rapid multiplication of the central segmentation-spheres, a, 

 by spontaneous longitudinal division, and, J, by the rather 

 rapidly advancing constriction of the apices of the segments ; 

 and, 2, a multiplication of the segments by their slower longi- 

 tudinal division. In this way, about the seventh hour of the 

 process of segmentation, there are produced 10-12 radiating 

 segments, whilst there are still only four central segmentation- 

 spheres ; in the eleventh hour there are eighteen segments and 

 at the same time fom'teen segmentation-spheres (eight by divi- 

 sion of the four above mentioned, and six newly constricted 

 apices of the two longitudinal and four lateral segments). In 

 the next (twelfth) hour a sphere is separated by constriction, 

 by means of the so-called meridional segmentation, from each 

 segment; all these spheres collect around those previously 

 formed, and consequently at this stage the number of seg- 

 ments amounts to eighteen, and that of the segmentation- 

 spheres to thirty-tAvo. In the next stage of segmentation the 

 number of segments increases to thirty-two, which surround 

 the germinal disk. But the latter now consists of 108-110 

 cells, larger towards the periphery, smaller in the centre, 

 which have multiplied in this manner by increased division. 

 The number of nuclei of the segmentation-spheres and seg- 

 ments likewise increases, a nucleus being contained in eveiy 

 sphere and in every segment. Both kinds of cells show no 

 trace of a membrane ; their finely granular protoplasm be- 

 comes constantly darker, and is transformed from a trans- 

 parent to a translucent substance. 



During the whole course of the process of segmentation the 

 outwardly directed surfaces of all the segments, and especially 

 of all the segmentation-spheres, are much raised, the highest 

 being placed in the centre of the formative vitellus. At the 

 close of the whole process, in the last stages, their convexities 

 are ft\r less observable ; and finally the tubercular surface of 

 the formative vitellus becomes quite smooth. As the final 

 result of all these divisions, the one-Jayered germinal disk 



