^ Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 



Fig. 26. (X 5-) 



Side view of embryo 24 days 22 hrs. old, length 10 mm, 23 — 24 pairs of myotomes. General 

 outline of body of embryo straighter, less curved laterally. Head and caudal extremities free from yolk. 

 Yolk becoming oval. Optic vesicles prominent. Ear better defined. Olfactory pits present. The mandibular, 

 liyoid and first branchial arches are distinct. The second and third branchial arches are not yet differentiated, 

 a slight process on the first branchial indicates the beginning of tlie gill bar. The anläge of the heart is 

 visible just beneath the arches. 



Fig. 27. (X 5.) 



Side view of embryo 26 days old , length 1 1 mm , 26—27 myotomes. General outline of body 

 straighter than in preceding stage. Head projects some 3 mm beyond margin of yolk; tail projects 1.2 mm, 

 is thinner laterally but broader dorso-ventrally. Eye, ear, nasal pits and mouth well defined. Maxillary 

 process discernible. Mandibular arches longer, but ventral ends widely separated. Second and third 

 branchial arches formed. Gill bars present on three branchial arches. Anterior limb buds indicated; faint 

 anläge of posterior limb buds. Yolk pear-shaped. Heart prominent. First surface capillaries present although 

 not indicated in figure. 



Fig. 28. (X 50 



Side view of embryo 30 days 8 hrs. old, length 13 mm, 30 — 31 myotomes. The trunk of the embryo 

 is nearly straight. At level of the posterior gill there is a pronounced neck bend and at the level of the 

 posterior limbs a striking downward bend of the tail. The epiphysis shows in surface views. The lens is 

 discernible. The ear is still vi.sible. The external nasal openings are sharply defined. The boundaries of 

 the mouth are better outlined owing to the approximation of the ventral ends of the mandibular arches. 

 The hyoid arch is becoming obscured. The gill bars are prominent on the three branchial arches. The 

 anterior limb buds project dorsally about .5 mm above the surface of the bod3\ The posterior limb buds 

 are but slight elevations. The yolk is pear-shaped with its dorsal surface much flattened. The auricular 

 and ventricular portions of the heart are apparent. The surface of the yolk is covered by a dense network 

 of capillaries which for the most part convey blood antero-ventrally to the abdominal vein. Considerable 

 pigment is present in the trunk region although but little has reached the outer portion of the dermis. 



Fig. 29. (X 5) 



Side view of embryo 36 days 16 hrs. old, length 16 mm, 36-38 myotomes. In general outline the 

 embryo shows a numljcr of striking changes. The neck bend is not so pronounced. The tail bend is 

 scarcely noticeable. There is a striking increase in dorso-ventral width of tail. The cerebral hemispheres 

 are well defined. The eye is now prominent and the lens better defined. The ear is no longer visible in 

 surface views. The mouth is well defined. The ends of the mandibular arches are closely approximated 

 but not united. The hyoid and branchial arches are more obscure. Anlagen of gill fiiaments present on 

 gill bars. Anterior limbs project dorsally. Posterior limbs are short ridges extending in horizontal plane. 

 The yolk is elongated and reduced in diameter both dorso-ventrally and laterally. Surface blood vessels 

 as in preceding stage, excepting that they are now apparent in the gill bars. The chromatophores are 

 most numerous in the anterior and dorsal portions of the head. 



