Ch. V] EARLY DEVELOPMENT OF THE EMBRYO 53 



the yolk-plug — the so-called opening of Rusconi, or blastopore 

 — is still smaller. The periphery of this circular blastopore is 

 Leeply pigmented. At 4.15, the opening is further reduced 

 jid measures no more than one-eighth of the diameter of the 

 >gg. The blastopore will now be found to have progressed so 

 ar that it again lies just beneath the equator of the egg, but on 

 die side of the egg opposite to that at which the dorsal lip first 

 appeared. We can summarize by saying that the dorsal lip of 

 die blastopore has moved over a meridian of the egg from a 

 >oint near the equator across nearly to the opposite point of the 

 jquator. The movement takes place over the lower white hemi- 

 sphere, and during the process the position of the egg remains 

 unchanged. The arc traversed by the dorsal lip of the blasto- 

 pore is, however, not as much as 180 degrees, because it started 

 below the equator and does not quite reach the equator at the 

 opposite side. But the arc is certainly more than 90 degrees, 

 and varies in different eggs. 



So far we have traced the history of the blastopore from six 

 o'clock in the morning to 4.15 in the afternoon of the same day. 

 Then a remarkable process begins. The blastopore moves back 

 as a whole in exactly the opposite direction until, at 7.45 in the 

 evening, it has come back to the point from which it started in 

 the morning. This reverse movement of the whole blastopore 

 is brought about by quite a different process from the first 

 movement of the dorsal lip. The ivhole egg rotates around a 

 horizontal axis. 1 



The overgrowth of the lower pole by the dorsal and lateral 

 lips of the blastopore has covered the lower hemisphere with 

 cells that do not contain as much pigment as do the cells that 

 lie around the upper pole, i.e. the original black hemisphere. 

 Hence when the egg rotates as a whole in the way just re- 

 corded, a somewhat lighter area will be carried into the new 

 upper hemisphere, while the original upper hemisphere will 

 noAV come to lie nearly on the lower side of the egg. In the 

 lighter upper region, as we shall soon see, the central nervous 

 system develops. The rotation of the whole egg appears to 

 take place through 180 degrees, although it is possible that the 



1 An axis at right angles to the median plane of the later embryo. 



