external gills 

 nasal pit 



, , , , myotomes 

 pronepnric bulge i 



TABl£ 7-1 A COMPARISON OF SOME OF THE FUNDAMENTAL 

 EMBRYOiOGICAL FEATURES DIFFERENTIATING THE 

 PROTOCHORDATES AND VERTEBRATES 



heart 



nasal pi' 



mouth 



eye 



^ oral sucker 



operculum external gills 



mouth 



remnant of oral sucker 



Figure 7-24. Some stages in the development of the frog larva. A, 

 at hotching; B, 8-mm larva; C, 1 1 '/2-mm larva; D, ventrol view 

 of head end of C to show beginning of opercular fold; E, opercular 

 fold covering right bronchial chamber with left chamber still opening 

 broadly behind it. (After Huettner, 1949) 



The larval forms of some tetrapods may be modified for 

 direct development. Fine examples are observed among the 

 frogs. The tail undergoes reduction, the limbs appear at a 

 very early stage, an operculum covering the gills may not 

 develop, and, in fact, the gills may not appear. 



The young of the frog is one of the most interesting verte- 

 brate larvae since it shows a certain parallelism to an as- 

 sumed primitive type. The agnathous mouth is armed with 

 horny teeth used for eating algae. Bony jaws develop later, 

 replacing the original mouthparts. There is an operculum- 

 enclosed branchial chamber which could be considered as a 

 kind of atrium. This atrium opens on the left side of the 

 body by a single opening. The asymmetry of this larva is 

 suggestive of that observed in the protochordates. 



The general opinion is that the frog larva is highly spe- 

 cialized rather than primitive. The fact that the anuran as 

 an adult is greatly modified does not necessarily imply that 

 its larva has not retained basic agnathous or even prea- 



Protochordate 



1 . Sperm enters near vegetal 

 pole. 



2. Mesoderm mostly from 

 enterocoelous pouches. 



3. Notochord directly of 

 archenteron wall origin. 



4. Blastopore converted by 

 ventral blastopore lip 

 growth into anterior 

 neuropere. Gut and neural 

 canal connected. 



5. Neural tube formed by 

 rolling upward of margins 

 from rear to front. 



6. Atrium formed by invagi- 

 nation of bilateral pouches 

 or outgrowth of bilateral 

 folds. 



7. Gastrulation by simple 

 invagination. 



8. No neural crest. 



Vertebrate 



1 . Sperm enters near animal 

 pole. 



2. Only a part of the meso- 

 derm of the anterior head 

 segments from enterocoe- 

 lous pouches. 



3. Only the tip of the noto- 

 chord indirectly from 

 archenteron wall. 



4. Blastopore closes; neuren- 

 teric canal of secondary 

 origin and short lived 



5. Neural lube formed from 

 groove which closes from 

 front to rear. 



6. .Atrium absent. 



7. Gastrulation only partly 

 due to invagination along 

 a crescentric, circular, or 

 straight line blastopore. 



8. Extensive sensory placodes 

 and neural crest develop- 

 ment in head and seg- 

 mental pairs of neural 

 crest ma.sses through full 

 length of body. 



gnathous features. Surely the algal food, reflected by the 

 highly coiled gut, the mouth parts and other features, could 

 be primitive. These features could just as easily represent a 

 more modified scheme than the relatively straight, simple 

 gut of the salamander larva. The observation that the 

 straight gut precedes the highly coiled gut does not solve 

 this problem, since it is necessary, as a matter of mechanics, 

 that a short tubular gut increase in length to give rise to a 

 coiled one. 



Although a study of larvae could certainly be e.xpanded 

 far beyond the above brief account, the resulting contribution 

 to the question of vertebrate phylogeny would be slight. 



RESUME OF EMBRYOGENESIS 



The vertebrates generally agree in their early stages of 

 development and show a marked similarity to the proto- 

 chordates in some features. These two groups differ most 

 notably in the ways summarized in Table 7-1. They agree 



RESUME OF EMBRYOGENESIS 



215 



