PRODUCTION OF VESSELS. 



529 



Production of vessels. 



Fig. 261. inal membrane of a fowl at thirty-sixth hour of in- 



cubation. (Wagner.) 



The formation of vessels from the coalescence 

 of nucleated cells, the touching ends becoming 

 pervious or elongating, is continued to a much 

 later period of development, as is demonstrated by 

 Fig. 262. 



Capillary lymphatic from the tail of the tadpole : #, membrane ; b, 

 Fig- 262. processes formed by it ; c, re- 



mains of the contents of the 

 cells forming these vessels, in 

 which nuclei are concealed; 0, 

 ccecal terminations of the ves- 

 sels ; jf, one of these termina- 

 tions still recognizable as a form- 

 ative cell ; g, isolated formative 

 cells about to join with actual 

 vessels, magnified 350 diame- 

 ters. (Kolliker.) 



It is at this tirns that nutri- 

 tion by cells ceases, and vascu- 

 lar nutrition commences, as pre- 

 viously described. The embryo 

 has now become too large for 

 promiscuous cell nutrition to an- 

 swer ; moreover, development is 

 required to take place at differ- 



Production of vessels) capillary lymphatic, magnified 350 Cnt rateS at isolated and Special 



points. The formation of the 

 amnion coincides with these events. 



The heart appears first as a canal or tube, arising in the vascular 

 layer from a columnar mass of cells, of which the inner ones Development 

 have deliquesced to form a tube. This then becomes tri- of the heart - 

 chambered, containing an auricle, a ventricle, and the bulbus arteriosus, 

 Fig 263 Fig. 263, of which a description is 



given on p. 135. Subsequently the 

 auricle and ventricle are each divi- 

 ded by septa, that in the ventricle 

 being commenced about the fourth, 

 and finished about the eighth week. 

 The auricular septum is not completed until after birth. 



Fig. 264, page 530, shows the human heart at about the fifth week : 

 A, the heart opened on the abdominal aspect; 1,. the bulbus arteriosus; 



LL 



Rudimentary heart. 



