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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



earlier position at the dorsalmost point of the 

 posterior. Those movements continue until the 

 nem-al rod occupies approximately two-thirds of 

 the circumference of the embryo. 



The union of the folds marks the beginning of 

 stage 11. The neural plate begins to round up 

 and appears in external view to have become a 

 neural tube with anterior and posterior neuropores. 

 This appearance is a result of the apparently simul- 

 taneous, progressive union of the folds in anterior 

 and posterior directions from the site of first 

 union, the middorsum. 



The external morphology of this stage is as 

 deceptive as that of the preceding stage since 

 histological examinations reveal that the neural 

 tube docs not possess a neurocoel; in reality the 

 neural tube is a neural rod as described by Shipley 

 (1885). In the anteromost region the neural rod 

 seems to elevate from the surrounding and under- 

 lying tissue. 



The end-point of this stage is reached when the 

 anterior region is raised above the globular yolk 

 mass. 



Stage 12: Head (fig. 18) days 6-S 



Size: 1.1 to 1.4 millimeters. 



Blastopore: Below posteriormost point of neural 

 rod, at ventral surface of embryo. Circular. 



Neural tissue: Very prominent from head to 

 blastopore. Neurocoel over presumptive pharynx. 



Head: Elevated from yolk mass. Free length 

 approximately 1.0 millimeter. 



Yolk : Globular as in preceding stages. 



Stomadaeum: Invagination begins. 



Measurements of the embryo from the tip of the 

 head to the posteriormost point of the yolk mass 

 along the neural rod gave a range from 1.0 to 1.1 

 millimeters when the head was just beginning to 

 form and 1 .3 to 1 .4 millimeters when the head was 

 fuUy formed and elevated from the yolk mass. 

 Although sections show that some somites have 

 formed, they are not discernible externally. 



The blastopore is below the posteriormost point 

 of the neural rod and faces directly ventral 

 during the initial period of this stage. When 

 the head is fully formed, the blastopore faces 

 anteriorly along the ventral surface toward the 

 head. 



Histological examination proves that the neural 

 rod forms its neurocoel over the presumptive 

 phar\nix only. By this time the neural rod ex- 

 tends along appro.ximately 75 percent of the 

 periphery of the embryo (fig. 19). The rod is 

 prominent and extends from the head to the blas- 

 topore. 



Figure 18. — Lateral view of stage 12, head. 



FiGnRE 19. — Parasagittal section of stage 12. 



Elongation takes place faster in the head region 

 than in the tail region. The anterior part becomes 

 well elevated and protrudes between the lateral 

 swellings. The length of the head and the pre- 

 sumptive branchial region is about 1.0 millimeter 

 at the end of the head stage. The appearance 

 of a head on the exterior of the embryo marks 

 the beginning of stage 12. This change is caused 

 by the sudden increase in length of the neural rod 

 during stage 11 and the sudden expansion of the 

 presumptive pharyngeal cavity both dorsally and 

 laterally to produce an upswelling of the pro- 

 spective head from its original position. 



