10. EMBRYOLOGICAL DEVELOPMENT 



107 



side the regular arrangement is somewhat disturbed and the difference in 

 size between the anterior and posterior cells is less sharp. In the centre of 

 this surface two median cells can often be distinguished, lying one in front of 

 the other. They stain more deeply than the surrounding elements, apparently 

 because of the nature of the yolk granules that they contain (Fig. 1SB). It is of 

 interest that the pair of large anterior cells of the concave side of the embryo 

 remain unchanged at this stage and their division has apparently ceased while 

 the remainder go on cleaving (Fig. 75$). 



msn 



Fig. 75. Gastrula of Siboglinum caulleryi at the 130— 140-cell stage: A -from the flattened side of 

 the embryo ; В - from the convex side ; С, D, E, F - transverse sections at the levels indicated. 

 ec - ectoderm; ma - large half-buried cell, lying near to the anterior pole; mi- anterior, median, 

 internal cell; mp - pair of posterior internal cells; msn- mesenchyme cells. (After Ivanov, 1957b.) 



In embryos of this age other cells besides those lying on the surface can be 

 distinguished. Large blastomeres lie wholly within the embryo occupying 

 an interstitial position. They resemble the pair of large cells which lie half- 

 submerged near the anterior pole on the concave side of the embryo (Fig. 

 755). Three massive cells amongst the internal elements catch the eye at 

 once, in sections; they occupy a large part of the anterior end of the embryo. 

 One of them lies in front near the convex side. This large median elongated 



