ASCIDIACEA CLEAVAGE. 



339 



cleavage," which was applied by Hatschek to the process in the 

 of Amphioxus, is equally suited to the very similar processes met 

 with iu the Ascidians. Certain characteristic irregularities are, indeed, 

 to be observed, and these are referable to the early foreshadowing of 

 certain important differentiations. 



The furrow which appears hist and divides the egg into two equal 

 halves corresponds to the plane of bilateral symmetry, and the two 

 blastomeres that result from this division represent the right and 

 left halves of the body (Seeliger, van Beneden, Julin and Castle). 

 From this stage onward through all the other stages of development, 

 the bilateral symmetry of the embryo is clearly recognisable. The 



Fig. 150. — Two stages in the cleavage of Clavelina (after Skkliger). A, four-celled 

 stage, seen from above. The two smaller cells v represent, according to Seeliger. 

 the anterior half of the body, and the larger cells h the posterior half. B, lateral 

 view of the eight-celled stage : "., blastomeres of the animal half; b, blastomeres of 

 the vegetative half. 



next furrow is also meridional and cuts the first at right angles. The 

 four blastomeres thus produced (Fig. 150 A) are not of exactly the 

 same size, two being larger (h) and two smaller (v).* According to 

 the most usually adopted orientation of the cleavage-stages in these 

 forms, the plane of cleavage now under consideration has a transverse 

 direction so that (according to van Beneden and Julin) the two 

 larger blastomeres represent the future anterior half of the body and 

 the two smaller the later posterior half, but Seeliger takes an 

 exactly opposite view (Fig. 149 A). In Distaplia, according to 

 Davidoff, the four blastomeres are exactly equal in size. 



*[In Ciona (Castle, No. II.) no difference in size is to be seen between the 

 blastomeres at the four-celled stage. The polar bodies are found to corre- 

 spond to the future vegetative pole. — Ed.] 



