kofoid: development of li.max. 75 



period in the seventh generation. The cleavages of the following gen- 

 erations arc meridional or equatorial, and belong to the bilateral pi 

 Another noticeable feature is the general precedence of the mesoderm- 

 producing quadrant d in the cleavages of the various quartets. 



The bilateral period in the cleavage of Umbrella is not so sharply- 

 marked off from the spiral period as it is in Nereis ; in Umbrella spiral 

 cleavage occurs as late as the ninth generation. In Nereis it ceases in 

 the seventh. The quartets 5.3 and 6.2 in both forms are the ones in 

 which the spiral character of the division first gives way to the merid- 

 ional and equatorial cleavage, — characteristic of the bilateral period. 



The cleavage of Umbrella, like that of Nereis, presents no contradic- 

 tions to the law of alternation of spirals. This striking agreement of 

 Nereis, Umbrella, and Limax must far outweigh any seeming contra- 

 diction arising in the work of the earlier writers upon spiral cleavage. 

 It is only necessary to apply the proposed system of nomenclature to the 

 careful work of Wilson and Heymons to make clear at once that the 

 alternation defended holds good. The system of nomenclature employed 

 in this paper facilitates the demonstration of the alternation of spirals 

 in successive generations of cells ; but the alternation itself is a factor 

 independent of mere names. It is the fundamental basis of the so 

 called " spiral type " of cleavage. A recognition of this fact might well 

 be embodied in nomenclature, and alternating cleavage substituted for 

 the ambiguous and misleading term " spiral cleavage." 



D- The Mesoderm. 



In the forty-four-cell stage, at which the discussion of the cleavage of 

 Limax w 7 as dropped, the germ layers are already differentiated. The 

 quartets 7.3, 7.4, 6.3, 7.7, 7.8, 6.5, 6.6, 6.7, 6.8, are pui-e ectoderm, the 

 quartet 7.1 and three fourths of the quartet 7.2, viz. a 72 , 6 72 , and c 7-2 , are 

 entoderm, while d 7 ' 2 is the sole representative of the middle germ layer. 



The seven entoderm cells and their progeny come to lie in the region 

 of the blastopore, and with the invagination are carried in to form the 

 lining of the archenteron. The primary mesoblast divides bilaterally, 

 i. e. in the median plane of the embryo, shortly after the forty-four-cell 

 stage. The two mesoblasts retain a slight connection with the exterior, 

 and at the ninety-cell stage have each divided transversely, the periph- 

 eral and posterior pair of cells are the smaller, and retain a slight con- 

 nection with the exterior. The next division occurs in the anterior 

 pair. The cells of the mesoderm continue to multiply until there are 



