EARLY DEVELOPMENT OF THE FROG. 65 



laterally the mesoderm extends between ectoderm and entoderm as pre- 

 viously described. Just behind the head region the paraxial mass suffers a 

 rearrangement of its cells so that a block is delimited transversely. Just 

 behind this another block is formed in the same manner; a similar process 

 produces a third, and so on toward the caudal region. The blocks themselves 

 consist of closely compacted cells while in the intervals between them the 

 cells are loosely arranged. These blocks are the mesodermal somites which 

 in their arrangement express the fundamental metameric or segmental prin- 

 ciple of all vertebrates and many invertebrates. Lateral to the somites a 

 cleft appears in the originally single layer of mesoderm thus dividing it into 

 two layers (Fig. 33). The cleft commences near the somites and gradually 

 extends all the way round ventrally; it also extends into some of the somites. 

 This cleft is the rudiment of the ccelom or body cavity, and its extension 

 into the somites probably corresponds to the myoccel in Amphioxus The 

 layer of mesoderm ectal to the ccelom and apposed to the ectoderm is called 

 the somatic or parietal layer; the layer ental to the ccelom and apposed to 

 entoderm is the visceral or splanchnic mesoderm. 



At this stage of development in the frog the fundamental vertebrate 

 organization is expressed in the general arrangement of structure (Fig. 37). 

 The body as a whole consists of a tube within a tube; the rudimentary diges- 

 tive system, extending lengthwise in the growing animal, is the inner tube, 

 the outer tube is the body wall, and the body cavity or ccelom separates the 

 two tubes. The dorsally located neural canal, the notocord around which 

 the vertebral column develops in the true vertebrate, and the series of meso- 

 dermal somites are at this time simple structural patterns from which the 

 complex vertebrate organization is evolved. 



References for Further Study. 



BRACKET, A.: Recherches sur Fontogenese des Amphibiens urodeles et anoures. 

 Archives de Biologie, tome 19, 1902. 



EYCLESHYMER, A. C.: The early Development of Amblystoma, with Observations on 

 some other Vertebrates. Journal of Morphology, Vol. 10, 1895. 



HERTWIG, R.: Furchungsprozess. In Hertwig's Handbuch der vergleichenden und 

 experimentellen Entwickelungslehre der Wirbeltiere. Bd. I, Teil I, Kap. Ill, 1903. Contains 

 extensive bibliography. 



JENKINSON, J. W.: On the Relation between the Symmetry of the Egg and the Sym- 

 metry of Segmentation and the Symmetry of the Embryo in the Frog. Biometrika, 

 Vol. 7, 1909. 



KELLICOTT, W. E.: Chordate Development. Chap. II, 1913. 



MORGAN, T. H.: The Development of the Frog's Egg. 1897. 



