FORMATION OF INTKA-EMBBYONIC CCELOM 



51 



paraxial mesoderm, each remains attached to the undivided lateral plate by 

 a continuous tract called the intermediate mesoderm or intermediate cell-mass 

 (fig. 73, III. c). According to Felix, this continuous tract is formed by the fusion, 

 At a very early stage, of the stalks of the segments. As the excretory ducts are 

 afterwards laid down in this tissue, it corresponds to those portions of the hollow 

 primitive segments which are named the nephrotomes in the Anamnia. 



A cleavage has meanwhile taken place in the lateral mesoderm, dividing it into 

 a parietal and a visceral plate. The parietal plate is associated with the ectoderm 

 to form the somatopleure, and the visceral plate with the entoderm to form the 



I 



FIG. 74. PHOTOGBAPH OP A CHICKEN EMBRYO, x 20 diameters. (T. H. Bryce.) 



The mesodermic segments, eleven in number at this stage, are seen as small cubical masses on each 

 side of the axis of the embryo. The eleventh is still continuous with the unsegmented axial mesoderm, 

 which in turn passes behind into the primitive-streak mesoderm. The neural folds have not united, 

 and they embrace posteriorly the head of the primitive streak. The optic vesicles are prominent 

 lateral projections from the fore-brain ; the mid-brain vesicle is visible behind the fore-brain, but that of 

 the hind-brain is hidden by the tubular heart, which receives posteriorly the two vitelline veins from 

 the vascular area. 



splanchnopleure. The space between the layers becomes the intra-em'bryonic 

 ccelom (body-cavity), and it follows that when the cleavage reaches the borders of 

 the shield the intra-embryonic will become continuous with the extra-embryonic 

 coelom, and the relations of the layers will be established which are reached at a 

 much earlier stage in lower mammals (fig. 77 ; cf. fig. 49, p. 33). 



The segments now also show a small cavity in their interior, round which the 

 cells arrange themselves in an epithelial fashion. The cavity represents a part of 

 the coelomic cleft, which in lower vertebrates is continuous with the general 

 ccelom. 



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