308 



CUCULLANUS. 



process which commences posteriorly, and then gradually extends for- 

 wards. In front the blastopore never becomes completely closed, but 



Sit ep 



iBiRi = =115' = =-- 



= 113 _"- -"-- --. -.-. = 



A. 

 B. 

 C. 

 D. 

 E. 

 F. 



FIG. 166. VABIOUS STAGES IN THE DEVELOPMENT OF CUCULLANUS ELEGANS. 



(From Butschli.) 



Surface view of flattened embryo at an early stage in the segmentation. 



Side view of an embryo at a somewhat later stage, in optical section. 



Flattened embryo at the completion of segmentation. 



Embryo at the commencement of the gastrula stage. 



Embryo when the blastopore is reduced to a mere slit. 



Vermiform embryo after the division of the alimentary tract into oesophageal 



and glandular divisions. 



m. mouth; ep. epiblast; hy. hypoblast ; me. mesoblast; a>. resophagus; bl.p. blasto- 

 pore. 



remains as the permanent mouth. The embryo after these changes 

 has a worm-like form, which becomes the more obvious as it grows in 

 length and becomes curved (fig. 1G6 F). 



The hypoblast of the embryo gives rise to the alimentary canal, 

 and soon becomes divided into an cesophageal section (fig. 166 F, 02) 

 formed of granular cells, and a posterior division formed of clear 

 cells. The mesoblast (fig. 166, me) takes its origin from certain special 

 hypoblast cells around the mouth, and thence grows backwards 

 towards the posterior end of the body. 



The young Cucullanus becomes hatched while still in the genera- 

 tive ducts of its parent, and is distinguished by the presence of a 

 remarkable thread-like tail. On the dorsal surface is a provisional 

 boring apparatus in the form of a conical papilla. A firm cuticle 

 enveloping the body is already present. In this condition it leaves 

 its parent and host, and leads for a time a free existence in the water. 

 Its metamorphosis is dealt with in another section. 



The ova of the Oxyuridte parasitic in Insects are stated by Galeb 

 (No. 386) to take the form of a blastosphere at the close of segmentation. 



