CHAPTER VII 



THE PRODUCTION OF ABNORMAL EMBRYOS WITH SPINA 



BIFIDA 



Embryos of the frog are occasionally found that differ 

 greatly from normal embryos. Roux, in 1888, first described 

 one of these embryos and showed that a knowledge of its 

 structure and method of development helped very much tow- 

 ard an understanding of the processes that take place in the 



A ^ B 



Fig. 27. — Two embryos formed as rings around equator of egg. A. Seen from in 

 front (produced in salt solution). (Morgan.) B. Seen from side. (After Roux.) 



normal development. An embryo described by Roux is shown 

 in Fig. 27, B. Around the equator of the egg along the zone 

 between the white and black hemispheres is a thickened ridge. 

 A careful examination shows that this ridge is not uniform in 

 thickness, but is bilateral in form. Each half is somewhat 

 thickened at one end, and resembles half of the medullary plate 

 of the normal embryo. Cross-sections (Fig. 29, B) show that 

 these ridges around the equator of the egg are the two halves 

 of the medullary plate. Instead, however, of being in close 



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