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NEURULATION AND EARLY ORGANOGENY 



Formation of the tail bud. 



tissue dorsal to the closed blastopore. As it is elongated it is provided 

 with both a dorsal and a ventral fin, the dorsal fin being developed 

 initially by the posterior growth of myotomes, with accompanying 

 blood vessels and nerves, to form the tail bud. 



The process of neurulation, or the formation of the central nervous 

 system of the frog embryo, is too complicated to understand by means 

 of verbal instructions alone. By means of time-lapse photography 

 these developmental changes can be telescoped into a few minutes and 

 understood more clearly. About the time the neural folds close, there 

 are numerous surface evaginations and invaginations which indicate 

 correlating changes within the embryo. The neurula develops surface 

 cilia which tend to rotate the embryo within the fertilization mem- 

 brane and its jelly (albuminous) coverings. 



Internal Changes 



The lining of the neural canal consists of the original pigmented 

 outer epidermal layer of the blastula which, by the time the canal 

 is formed, is ciliated, as is the entire outer ectoderm of the embryo. 

 The central canal of the nervous system, when it is formed, is there- 

 fore lined with ciliated and pigmented cells, later to be identified as 

 the ependymal layer. The bulk of the nervous system, however, is 

 derived from the inner layers of polyhedral cells from the original 

 nervous layer of ectoderm of the blastular roof. This properly named 



