Normal Plates of tlie Development of Necturus maculosus. 3 



Neural groove extends to transverse portion of fold but does not reacli blastopore. Segmentation cavity 

 no longer visible in surface views. 



Fig. 18. (X 10.) 

 Top view of embryo 17 days 2 hrs. old. Blastopore an elongated narrow aperture between ends 

 of neural fold. Neural plate narrower than in preceding stage. The constricted portion represents in 

 a general way the division between head and trunk. Neural fold most prominent in head region. 



Fig. 19. (X 10.) 

 Top view of egg 17 days 17 hrs. old. Blastopore no longer visible. Neural plate narrowest poste- 

 riorly; broad in head region, showing boundary zone between head and trunk. Lateral portions of fold 

 coalesced at posterior end of embryo. At anterior end of embryo a deep groove partially separates the 

 two halves of the neural fold. 



Fig. 20. (X 10.) 

 Top view of egg 18 days 13 hrs. old. Lateral portions of neural fold almost united except in 

 head region where they are still widely separated. In the antero-lateral portions of the fold are slight 

 evaginations which are the beginnings of the optic vesicles. 



Fig. 21. (X 10.) 

 Top view of egg 18 days 15 hrs. old, 3 or 4 pairs of myotomes. Lateral portions of neural fold 

 widely separated in head region, more closely approximated in anterior trunk region, coalesced in taii. 



Fig. 22. (X 10.) 

 Dorso-lateral view of embryo 20 days 10 hrs. old, length 6 mm, 6 pairs of myotomes. Outline of 

 body conforms to curvature of egg. Head end of embryo shows three longitudinal ridges; middle ridge 

 lies slightly above level of lateral ridges. The middle one is common anläge of fore, mid and bind brain. 

 The lateral ones are the common anläge of the optic vesicles and branchial arches. Anus formed. 



Fig. 23. (X 10.) 

 Side view of embryo 21 days 2 hrs. old, length 7 mm, 10 — 12 pairs of myotomes. General outline 

 of body conforms to curvature of egg. Head slightly raised above surface of yolk. Slight enlargement at 

 end of tail. A distinct enlargement of anterior end of head shows optic vesicles; just posterior to this 

 enlargement is the anläge of the branchial arches. Anus shows just below tip of tail. 



Fig. 24. (X 10.) 

 Dorso-lateral view of embryo 22 days 17 hrs. old, length 8 mm, l6~i8 pairs of myotomes. Embryo 

 much curved laterally. Anterior half of head free from yolk. Caudal enlargement more prominent. Optic 

 vesicles and mandibular arch well defined. The hyoid and first branchial arches are discernible ; also the 

 common anläge of the second and third branchial arches. 



Fig. 25. (X 5-) 



Side view of embryo 23 days 10 hrs. old, length 9 mm, 20 — 22 pairs of myotomes. General outline 

 of the body straighter. Head free from yolk. Caudal enlargement becoming free. Optic vesicles and 

 forebrain much larger. Mandibular, hyoid, first branchial, and common anläge of second and third branchial 

 arches well defined. Otic vesicle visible above hyoid arch. 



