Mosaic Devclopmejit m Ascuitan Eggs. 169 



positions. Furthermore these sense spots may be more numerous 

 than in the normal larva, as shown in Figs. 45 and 46, probably 

 owing to the fact that the cells which form the pigment and which 

 normally lie on the margins of the neural plate do not come to- 

 gether to form two spots as in normal larvae, but remain separated 

 so that several such spots are formed. 



(2) Notochord. The chorda cells grow back into the posterior 

 half of the embryo and the cells here interdigitate in the normal 

 manner, finally forming a linear series of cells. (Figs. 32-46.) 

 The notochord, which is at first relatively short and thick. Fig. 33, 

 becomes later very much longer and more slender. Fig. 40, and in 

 all respects it has the appearance of a normal notochord, save 

 that it evidently contains a smaller number of cells. The position of 

 the notochord of the half larva is always slightly abnormal; it never 

 lies along the original median plane (first cleavage) as in normal 

 larvae, but its anterior end is diverted aw^ay from that plane and 

 toward the lateral border of the larva. (Figs. 32, 33, 37, 41.) This 

 position is that which the chorda cells, which arise in the anterior 

 lip of the blastopore and which grow posteriorly around the mar- 

 gin of the blastopore, would naturally assume, (f/. Figs. 31 and 33.) 

 What it is which causes the chorda cells to interdigitate in their 

 characteristic manner is a question difficult to answer; it certainly 

 is not dependent upon the crowding together of chorda cells from 

 the right and left sides since it occurs normally when the cells of 

 one side only are present; on the other hand it must depend upon 

 a certain amount of lateral compression of the chorda cells since 

 it occurs very rarely if at all in the anterior half larvae in w^hich the 

 ectoderm and mesoderm of the tail are lacking. 



(j) Muscles and Mesenchytne. In these right or left half 

 embryos and larvae the muscle and mesenchyme cells are present 

 on one side of the notochord; here they occupy their normal posi- 

 tions, the muscle cells givmo; rise to three rows of cells along the 

 lateral border of the notochord and the mesenchyme forming a 

 group of small cells anterior to the muscle rows. (Figs. 32, 33, 36.) 

 In later stages the muscle cells slowly extend over to the side of the 

 tail on which they were originally lacking; this takes place espe- 

 cially at the hinder end of the tail, the overgrowth taking place 

 around the end of the notochord and over its ventral side. In this 

 way the right or left half embryo or larva tends to become com- 

 plete, but I have never seen a case m which three rows of muscle 



