28 ART. 1. ï. FUJIÏA : ON THE FORMATION 



middle portion of the egg. This latter is intended to illus- 

 trate the mesomere after its complete division into two daughter- 

 cells and after their full accommodation. The three entomere-cells 

 A, B, and G now commence to divide as in Fig. 25. In this case 

 their nuclei do not show any shifting, such as was seen in Aplysla. 

 The entomere-cells are divided into two lialves almost equal in size ; 

 and all are arranged on the same level. From this time onward 

 the entomere-cells dispose themselves in a layer and may now l)e 

 called the entoderm -layer. 



The further developmental course of the germ-layers has 

 been studied with some degree of exactness only in Siphonaria. 

 The ectoderm-cells gradually begin to divide indiscriminately ; and 

 hence it is wholly impossible to trace them in surface views beyond 

 the tenth generation. The ninth quartet (^<2.2.b '^2.'2.i5 etc.), arises 

 from the sixth generation of ectoderm-cells (^i;^ h,2, etc.), and 

 the tenth («2.1.1.1, ^^2a.\a, etc.), from the seventh («2.1.1, '^2. 1.1, etc.), (Fig. 

 30). In what quadrant they first appear, and how the courses of 

 cleavage run are quite uncertain. The new daughter-cells are 

 always as small as their mother-cells. 



The entoderm-cells iro through their second cleavage, at 

 the same time with the second division of the mesoderm-cells. This 

 process begins with the cell A^^ and proceeds in a right-handed 

 spiral, as seen in Fig. ^34, a horizontal section of this stage, in 

 which A^ and i)i have given off respectridy Ai,^, and i)i.i, and 

 C\ shows a spindle. The small daughter-cells thus produced 

 exactly alternate in position with their original mother-cells. 

 The subsequent growth of the entoderm-cells seems very slow, and 

 I have not traced it Iteyond this stage. I will only add here that 

 the first entoderm-cells retain their original relative positions' for 

 a long time as indicated l)y the persistence of the cross furrow. 



