section of a very young bud, ind sho'-7s tne rudiment 

 wrien it consists merely of a few cells loosely at- 

 tached to the inner vesicle. The remaining fig- 

 ures are drav/n from transverse sections. In h and 

 c, the rudiment has increased in size, cells are 

 being added from without, and nuclei v/ithin the 

 mass are seen to be dividing. Tn d and e^, a cav- 

 ity has appeared in the centre of t}ie rudiment, 

 which is now much larger, and the cells are becom- 

 ing arranged into a one-layered epithelium. .■' . 2. 



Fig. 28. Transverse section through tne anterior end of a 

 biid slightly older than Pig. 9. The rudiment of 

 the dorsal tube is shown, and consists at this 

 stage of a few scattered cells, adhering to the 

 wall of the inner vesicle a little to the left of 

 the median dorsal line. Tne difference in thick- 

 ness between the right side and the rest of tne ves- 

 icle is seen to be but slight in t.iis region. D.2. 



Fig. 29, a, b, c, d, and_e . Transverse sections representing 

 stages in the development of the dorsal tube. In 

 a a fevf cells are seen loosely grouped together and 

 lying on trie wall of the inner vesicle. In b^ the 

 association witn surrounding mesodermal cells is 

 apparent. In c and d the mass has become compact, 

 and cell-boundaries have entirely disappeared; in 

 the latter figure the I'omen in the centre has begun 

 to form. In e tne tube is completely formed, and 



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