EMBRYOLOGY OF THE SEA LAMPREY 



121 



Figure 14. — Dorsal view of stage 10 illustrating neural 

 groove. 



Figure 15. — Posterior view of stage 10 illustrating blasto- 

 pore and neural plate with neural groove. 



blastopore is now at the uppermost point of its 

 apparent migration along tlie mid-sagittal plane 

 and the flattened ectodermal cells (neural plate) 

 are at the height of their thickening. Practically 

 all the vegetal cells are covered by tlie epibolizing 

 animal cells. Immediately after the neural plate 

 thickens, the central portion of the plate begins to 

 form a trough, producing a neural groove and 

 fold stage (fig. 16). As tlie groove in the neural 

 plate deepens the "folds" begin to approximate 

 each other and unite. The first actual union 

 occurs in the mid-dorsal region of tlie embryo. 

 This union is the end-point for stage 10. 



Figure 16. — Cross-section of stage 10 

 Stage 11: Neural rod (fig. 17) days 5-6 



Size: 1.1 to 1.3 miUimeters. 

 Blastopore : Circular. Apparent migration 

 toward ventral surface of embryo. 



Figure 17. — Oblique lateral view of stage 11, neural tube. 



Neural tissue: Union of folds middorsally on 

 embryo. Neural tube lacks netirocoel, and thus 

 is a neural rod. Becomes prominent across dor- 

 sum of embryo; circumscribes approximately two- 

 thirds of embryo. 



In the late phase of this stage the dorsopos- 

 teriorly located, circular blastopore begins an 

 apparent migration toward the ventral surface 

 accompanied by a similar ventral movement of 

 tlie prospective head. Toward the end of tliis 

 stage the circular blastopore is at the venti'al- 

 most point of tlie posterior as contrasted witli its 



