MUSEUM OF COMPARATIVE ZOOLOGY. 253 



resembles that of the protoplasmic bridge of the primary furrow, my 

 explanation is probably erroneous. 



The large lobe is the point of origin of a new cleavage furrow, which 

 I have called the tertiary or third cleavage furrow (3 d. pi.). In the 

 general structure and mode of origin the third cleavage furrow bears a 

 striking likeness to the primary and secondary. It forms at right angles 

 to the direction of the second furrow and parallel with a part of the 

 first furrow in the large undivided lobe on the left-hand side. At 3 h. 

 5 m. p. M. (PI. II. fig. 2) the tertiary furrow had not begun to appear; 

 but ten minutes after, at 3 h. 15 m. p. M. (PI. II. fig. 3), it had reached 

 a considerable size. Like the primary and secondary furrows, the walls 

 of the tertiary are formed by an infolding of the surface of the ovum, 

 and have the characteristic sharply defined folds and plications already 

 mentioned. 



Figures of the egg at 3 h. 20 m. p. m. (PL 11. fig. 4), and at 3 h. 

 25 m. p. M. (fig. 5), are inti-oduced in order to show the progress of the 

 growth of the tertiary furrow in the division of the large undivided 

 lobe on the left-hand side of the Qgg. At 3 h. 30 m. p. m. (fig. 6) two 

 hours and a half after the formation of the first cleavage furrow, the 

 tertiary furrow has divided this lobe horizontally into two smaller cells. 

 The portion of the tertiary plane which bisects the large lobe is, like 

 the primary and secondary, perpendicular to the plane of the paper on 

 which the egg is figured. The two axes of the e^g, a vei'tical, which is 

 the original cleavage plane, and the horizontal, the secondary plane, are 

 easily distinguished, and at one end of the tertiary furrow, now almost 

 completely closed in, there is figured a marked protoplasmic elevation. 

 This stage is a 6-cell stage, composed of the four cells which have 

 already been mentioned and the two additional which have just formed. 

 The tertiary furrow was the third furrow observed, but I suspect that 

 between the secondary and tertiary (by my nomenclature) the large lobe 

 which I have represented as divided by this furrow was constricted from 

 the two left-hand cells by another, whose growth was not observed. 



Morula. 



The complications in the growth of the ovum after the stage last 

 mentioned make it very diflicult to follow the birth of new segment 

 spheres or cleavage planes. The last stage of the egg in which the 

 course of the original cleavage can be traced with any certainty is at 

 3 h. 45 m. p. M. (PL II. fig. 7), or two hours and three quarters after the 



