IV. i INITIAL STRUCTURE OF THE GERM 165 



Since the crescent is formed on the side opposite to the entry 

 of the spermatozoon, the latter is considered by Roux to 

 determine the symmetry -plane of the egg. The first furrow is 

 also stated to generally lie in this plane, and the dorsal lip of 

 the blastopore, which marks, of course, the sagittal plane of the 

 embryo, appears in the region of the grey crescent. The dorsal 

 lip grows through 75, starting 25 below the equator and 

 passing beyond the vegetative pole; with the rotation of the 

 whole egg and therefore also of its axis in the direction opposite 

 to that through which the dorsal lip has travelled, the anterior 

 end of the embryo, whose dorsal side is now uppermost, comes to 

 lie a little above and behind the animal pole, while its posterior 

 end is marked by the now fast-closing blastopore (Fig. 79). 



According to Roux the first furrow and the sagittal plane 

 coincide and the first two blastomeres are right and left, unless, 

 by an anachronism, the second furrow, which separates anterior 

 from posterior (this should be dorsal from ventral), occurs first. 

 The third furrow, therefore; separates dorsal from ventral (cor- 

 rectly speaking, anterior from posterior). In the small number 

 of eggs examined by Roux the percentage of coincidences was 

 found to be high (80 %) and the deviations were attributed to 

 experimental error. 



Oscar Hertwig has, however, stated on the strength of a 

 small number of observations on eggs compressed between hori- 

 zontal glass plates that the angle between the two planes may 

 have any value; while Schulze and Kopsch think it probable 

 that they coincide in the majority of cases. The recent exami- 

 nation by the present author of a large number of cases has 

 shown that the angle in question may indeed have any value 

 from to 90, but that there is a decided tendency to small 

 values, that is to coincidence, as may be seen from the annexed 

 frequency polygon (Fig. 80), although at the same time there is 

 no correlation, and therefore no causal connexion, between the 

 two planes. Between the plane of symmetry and the sagittal 

 plane, however, the correlation is considerable, and the tendency 

 of the two to coincide much greater (Fig. 81), a result in con- 

 formity with the statements of other observers (Roux, Schulze, 

 Morgan). Within certain limits, therefore, the right and left 



