EMBRYOLOGY OF THE SEA LAMPREY 



123 



The yolk mass retains the globular shape it had 

 in the preceding stage. The stomodaeal invagi- 

 nation begins in the ventral portion between tlie 

 lateral swellings. 



The end-point of this stage is reached when the 

 head region begins muscular activity. 



Stage 13: Prehatching (fig. 20) days 8-12 



Size: 1.4 to 2.5 millimeters. 

 Blastopore: Circular. Much reduced. Lo- 

 cated at anteriormost point of apparent migration. 



Figure 20. — Lateral view of stage 13, prehatchiiig. 



Neural tissue: Neural rod now a true neural 

 tube. Prominent above somites from blastopore 

 to head. 



Head: 2.0 millimeters. 



Yolk: Obovate, blunt end posterior. 



Stomodaeum: Deepens and widens. 



Somites: 5 to 20; not easily distinguished 

 externally. 



Locomotion: First muscular activity. Move- 

 ment of free embryo, head. 



The length of the head region has increased 

 from 1.0 millimeter at the beginning to 2.0 milli- 

 meters at the end of this stage. Measurements 

 were taken from the tip of the head to the anterior- 

 most part of the yolk mass. 



The blastopore, which is situated antero- 

 ventrally lies at its anteriormost position directly 

 opposite the stomodaeal invagination. 



Stage 13 begins with the advent of muscular 

 activity. These movements initially are mere 

 lateral flexions of the head and "neck" portion of 

 the embryo; up-and-down activity is not apparent. 

 As the embryo enlarges, muscular contraction 

 includes dorsoventral flexion and becomes \n\- 



566128 O — 61 3 



dulating rather than wagging. With increase in 

 in the embryo's size the perivitelline space be- 

 comes fully occupied, since the embryo, arranged 

 in circular fashion within the membrane, begins 

 to spiral upon itself. During this growth the 

 above-mentioned movements become more forceful 

 and more frequent. 



Embryos that show movement but still have in- 

 tact fertilization membranes are in stage 13. The 

 endpoint of stage 13 comes when the head pro- 

 trudes through the fertilization membrane to 

 initiate hatching. 



Stage 14: Hatching (fig. 21) days 10-13 



Size: 3.0 to 5.0 millimeters. 



Blastopore: Minute opening at apex of 90° 

 ventral flexion of posterior. Dubious structure for 

 staging. 



Figure 21. — Lateral view of several prolarvae of stage 14, 

 hatching, showing posterior curvature, somites, yolk 

 mass, and condition of mouth and nostril. 



Neural tissue: Neural tube still very prominent 

 above somites. 



Yolk (gut) : Becomes slender and assumes a 

 spatulate shape. Posterior region has 90° ventral 

 flexion. Anteriormost portion greenish. 



Stomodaeum: Deep pit located ventrally. 



Somites: 18-20 to 30-35. Extend from neural 

 tube to gut. 



Locomotion: Hatching movements. Undula- 

 tion of anterior bodv region onlv. 



