10. EMBRYOLOGICAL DEVELOPMENT 



109 



by their dimensions, position, special yolk content, natural colouration and 

 staining reactions in sectioned material. In other words, there is a point of 

 sharp differentiation of the blastomeres giving the cleavage a determinate 

 character. On that account one cannot separate early development into 

 distinct periods of cleavage, gastrulation and early organogenesis. 



As for gastrulation, all that can be stated with any certainty is that it does 

 not take place by either immigration or invagination. The peculiar internal 

 cells, which later give rise to the archenteron and coelom, apparently arise by 

 means of cleavage within a compact morula or sterroblastula. It may be that 

 there is some degree of overgrowth of the larger cells by the smaller at this 

 point, i.e. of epiboly. 



Formation of the coelom 



Soon the embryo increases a little in size and its front end becomes 

 somewhat broader. Formation of the ectoderm is now completed. Near the 

 anterior end a rather prominent girdle is distinctly visible consisting of two 

 transverse rows of larger ectodermal cells (Fig. 16Л). A similar but narrower 



сое I 



vac 



сое II -III 



сое II -HI 



Fig. 76. Formation of the coelom in Siboglinum caulleryi - A -embryo from the flattened side; 

 В and С - transverse sections at the anterior end and through the centre of the embryo respectively. 

 a - cells of the anterior ectodermal girdle; ar - archenteron; сое I- primordium of anterior coelom; 

 сое II-III - lateral pouches - common primordia of the second and third coelomic cavities ; 

 ec - ectoderm; vac - vacuoles representing the site of fat droplets. (After Ivanov, 1957b.) 



