246 bulletin: museum of comparative zoology. 



(c' ^', d''-^^, Fig. 71), one in the right and oue in the left half of the 

 embryo. They are now separated both from each other and from the 

 anterior chorda cells. We shall see later how they are brought into 

 contact with each other, in the median plane, and with the anterior 

 chorda cells. 



The mesenchyme mother cells are also ten in number, but, unlike the 

 chorda cells, they are derived chiefly from the posterior quadrants. They 

 are A^-^"^, its deep-lying sister cell, ^^•", d'"^-, D'-^, and UP-^, with tlie 

 corresponding cells in the left half of the embryo, all indicated by a flat 

 tint in the Figures.^ It will be observed that the mesenchyme fundament 

 is made up of cells derived from both hemispheres and all four quadrants. 



The outer of the two rows of cells encircling the endoderm fundament 

 will be called the neuro-muscular ring. (Fig. 71. The cells are stippled.) 

 It is interrupted at three points \)y mesenchyme cells of the inner ring; 

 in the middle line behind, by the small flattened cells, C'^, D'-^ ; on the 

 right side, by A^-^'^ ; and on the left side, by B^'^^. It is thus divided 

 into three portions, an anterior segment of eight cells, all descended 

 from the anterior quadrants, and two latero-posterior segments, each 

 composed of four cells, descended from one of the posterior quadrants. 

 The anterior segment is composed purely of nerve mother-cells, which will 

 form a considerable portion of the medullary plate. The other segments 

 will form the entire longitudinal musculature of the larva, as well as a 

 certain portion of the nervous system in the tail region. 



In the two rings of cells just described are included all save two of 

 the descendants of the cells forming the equatorial hand of the 48-cell 

 and later stages. These two cells are Z)^-^^ and C^•^^ situated at the 

 posterior margin of the embryo (Fig. 71). They form, in my opinion, 

 definitive ectoderm. 



The remaining cells of the embryo number sixty-four, all descendants 

 of the ectodermal group of the 48-cell stage. They will form definitive 

 ectoderm, possibly also a portion of the medullary plate. 



One again notices in this stage the striking difference in rate of di- 

 vision of the cells which he meets in passing from the vegetative toward 

 the animal pole, a difference which made itself apparent as early as the 



1 Samassa ('94) identified the mesencliyme mother cells D'"^ and c?'i- (the cells 

 8 and 9 of his Fig. 10) as nerve cells. In my preliminary paper I expressed a dif- 

 ferent opinion, stating that they were mesoderm cells. Subsequent study has 

 confirmed this view, but shown that I was wrong in stating, as I did, that they 

 would contribute to the formation of " the longitudinal musculature of the tail." 

 That organ has, as I shall show, an entirely different and hitherto unsuspected 

 oritfiu. 



