1 68 Edivm G. Conklin. 



c. Formation of Larva. 



A considerably later stage in the development of the half embryo 

 is shown in Figs. 32, 33 and 36 (Figs. 34 and 35 are three-quarter 

 embryos and will be described later); of these stages Fig. 33 is the 

 youngest and Fig. 36 the oldest. In all ot these figures the blasto- 

 pore has already closed and the chorda cells have given rise to a 

 fusiform notochord, which lies in the posterior half of the embryo. 

 The blastopore closes chiefly by the posterior growth of the dorsal 

 (anterior) lip, as in the normal gastrula. With the formation of 

 the notochord the posterior half of the embryo becomes elongated 

 and narrower than the anterior half and the developing tail bends 

 around toward the injured side. (Figs. 32, 33.) 



The anterior half remains large, the posterior half becomes long 

 and narrow; the latter portion contains the notochord and muscle 

 cells, the former the gastral endoderm, mesenchyme and most of 

 the neural plate. The general superficial appearance of an 

 embryo of this stage is very similar to a normal one, but a more 

 detailed study shows many differences. 



(j) Neural Plate. The neural plate occupies in the main its 

 normal position, that is, it lies along the first cleavage plane on the 

 dorsal side, next to the injured cell. In this position the plate 

 becomes folded and ultimately comes to contain a vesicle (the 

 sense vesicle) though the steps by which this vesicle is formed are 

 always irregular and abnormal. (Figs. 36-40.) The anterior por- 

 tion of the plate is usually doubled over posteriorly while the 

 posterior portion is folded forward (Figs. 36, 39, 40) and in this 

 way a vesicle is finally formed. 



The tail of the embryo grows around toward the injured side 

 so that the concave side of the embryo is median or dorsal, the 

 convex side being lateral or ventral. In the younger, normal 

 larvae the concave side is ventral, the convex dorsal. In these 

 half larvae the nerve plate lies along the concave side, a con- 

 dition which is the reverse of what is found m the normal larva. 

 (f/. Figs. 12 and 36.) In the older half larvae there is almost 

 always found one or more pigmented sense spots m the neural 

 plate or sense vesicle. (Figs. 36-40, 45, 46.) These pigment spots 

 appear within cells of the neural plate and, as I am well convinced, 

 always within definite cells, though owing to the abnormal foldings 

 of the neural plate they do not always occupy exactly the same 



