356 E YCLESH YMER. [Vol. X. 



One furrow of the second set may extend at right angles to 

 the first, while the other forms an acute or obtuse angle, or 

 both may form acute and obtuse angles as shown in Figs. lo 

 and II. Both furrows of the second set may fall far to one 

 side of the upper pole, forming two larger and two smaller 

 cells (Fig. 9). One of the furrows may reach the opposite 

 pole before the other has started, or it may not appear at all 

 as a vertical but as an equatorial; such a case was observed in 

 the living eggs shown in Figs. 5 and 6. 



Third cleavage. — The third cleavage occurs in about i hr. 

 and 40 min. after the second. This set is often horizontal, 

 occasionally vertical, again a part vertical, a part horizontal, 

 and others which fall between the two so that it becomes im- 

 possible to select a type. A few of the more frequent varia- 

 tions are recorded. 



A variation is shown in Fig. 10, where three blastomeres 

 are cut off by horizontal furrows, while in the remaining quad- 

 rant no furrow is yet formed, although one of the fourth set (4) 

 has appeared. 



In Fig. 7, horizontals (3) are present in two quadrants only, 

 giving rise to two smaller micromeres ; while a third furrow 

 (3) passes vertically, taking a direction and position which 

 would permit its interpretation as one of the second set. 

 Again, in the Q%'g shown in Figs. 3 and 4, three quadrants are 

 divided by verticals, while in the fourth a " tangential " 

 (Kupffer, '9o) is formed. In Figs. 12 and 13 we observe a set 

 of verticals, and an entire absence of horizontals. 



The variations recorded may offer a solution to what has 

 hitherto given rise to no little perplexity concerning this cleav- 

 age. The majority of observers maintaining that this cleavage 

 is horizontal while McClure ('93) holds the typical method to be 

 the formation of a set of verticals followed by horizontals. 



The succeeding cleavage is represented in Figs. 14 and 15. 

 The directions of the furrows are so varied that their classifica- 

 tion is impossible, as is evident not only from my own observa- 

 tion but also from the fact that scarcely two observers agree 

 as to its nature. Max Schultze ('56) held that two more equa- 

 torials follow the third. Shipley ('88) states that it is followed 



