240 GENERAL EMBRYOLOGY 



or centrolecithal ova (meroblastic) . Here little or no geometric 

 regularity of cleavage pattern can be made out. We may add 

 a few details concerning discoid and superficial cleavage to the 

 brief statements made on a preceding page. 



Discoid. This is chiefly characteristic of the Craniata but it 

 is found occasionally in the Arthropods (Scorpions). In the 



c ,:,:? 



s.c. 



FIG. 116. Cleavage in the sea-bass, Serranus atrarius. From H. V. Wilson. 

 A. Surface view of blastodisc in two-cell stage. B. Vertical section through 

 four-cell stage. C. Surface view of blastodisc of sixteen cells. D. Vertical 

 section through sixteen-cell stage. E. Vertical section through late cleavage 

 stage. c.p. t central periblast; ra.p., marginal periblast; s.c., segmentation cavity 

 (blastocoel). 



ova of these forms there is often a fairly definite demarcation 

 between the protoplasmic and deutoplasmic portions (Elasmo- 

 branchs, Teleosts, Reptiles, Birds) and the cleavage planes are 

 practically limited to the former region, known as the blasto- 

 disc (Figs. 48, 157-159). The early cleavages may be fairly 

 regular, and approximate either radial or bilateral arrange- 

 ments, and as far as the protoplasm alone is concerned, the 



