462 BERTRAM G. SMITH 



respective points of origin, the first cleavage furrow has reached 

 the lower pole where its ends unite. The first cleavage furrow 

 thus becomes superficially complete, thereby establishing the 

 holoblastic character of the egg. That there is a strong mero- 

 blastic tendency is already apparent. In the region of the vege- 

 tal pole the first cleavage furrow is at first broad, but it later 

 becomes narrow and faint. 



With very few exceptions, the third cleavage furrows depart 

 from the second furrow, at some little distance from its point of 

 intersection with the first. In a previous paper (Smith '06) 

 it was erroneously stated that the third cleavage furrows usually 

 depart from the first furrow. 



The third cleavage furrows ordinarily begin as two pits in 

 the second furrow, equidistant from its point of intersection at 

 the animal pole. From these two pits the third cleavage furrows 

 ordinarily proceed in an approximately vertical direction (fig. 

 65), and do not become complete until a later stage (Stage 5). 



From the time of the earliest appearance of the third cleavage 

 furrows, the distances from the first cleavage furrow to their 

 points of departure from the second remain unaltered; but the 

 second cleavage furrow, originally straight, often becomes drawn 

 into a zig-zag line, as shown in the figures. 



As will be seen from a study of later stages, the third cleavage 

 furrows rarely reach the vegetal pole, but as a rule extend obliquely 

 in the lower hemisphere to join the^rs^ furrow at some distance 

 from the lower pole (figs. 84 and 91 to 96). Hence the general 

 statement may be made that the third cleavage furrows are inter- 

 mediate between a true meridional and a true latitudinal cleavage 

 but approach more nearly to the former type. 



In the typical condition, the cleavage pattern has now lost 

 its strictly radial, and acquired a biradial symmetry. I have 

 purposel}!" avoided the use of the word bilateral in this connection, 

 not only because it does not fit the case so well as the word bi- 

 radial, but in order to avoid the inference that the condition has 

 anything to do with the bilateral symmetry of the future embryo. 

 As will be seen by consulting the figures, this biradial conditfon 

 of the cleavage pattern persists in the lower hemisphere through- 



