PLATYELMINTHES. 



the nearly closed blastoporic area to four small cells. In con- 

 nection with these cells a complete hyploblastic wall becomes 

 subsequently established, which encloses the original large hypo- 

 blast cells. The latter then become resolved into a vitelline 

 mass. 



From a comparison with other types it may be regarded as 

 probable that the enteric wall originates by a process of continu- 

 ous budding off of small cells from the large cells, which com- 

 mences with the formation of the four cells above mentioned. 



The blastopore becomes nearly obliterated, but whether it 

 gives rise to the mouth, which is formed in the same place, has 

 not been determined. In front of the mouth a small and very 

 transitory rudiment of an upper lip makes its appearance. The 

 protrusible pharynx is stated by Hallez to arise as an hypoblastic 

 bud, while its sheath has an epiblastic origin. Two pairs of 

 eyes and the supra-cesophageal ganglia also become early 

 developed. 



The peripheral ciliated layer of small cells becomes divided 

 into two strata, of which the outer remains ciliated and forms 

 the true epiblast : the inner probably forms the cutis. In it are 

 developed rod-like bodies, which seem to be homologous with 

 the thread cells of the Ccelenterata, so that if the views put 

 forward in the previous chapter as to the similarity of the turbel- 

 larian and ccelenterate larvae are correct, the cutis corresponds 

 with the deeper layer of the coelenterate epiblast. The meso- 

 blast, like the epiblast, becomes divided into two strata. The 

 outer one is stated to form the circular and longitudinal muscles; 

 the inner one to give rise to a muscular reticulum, the spaces 

 within which constitute the parenchymatous body cavity. 



The later changes are not of great importance. At a period slightly 

 after the formation of the mouth and ganglia two pairs of stiff hairs become 

 formed at the sides of the body. The embryo has by this time grown so as 

 to fill up its capsule, in which however it continues rapidly to rotate, and also 

 commences to exhibit active contractions. It next becomes hatched, and 

 passes from a spherical to a flattened elongated form. The ventral oral 

 opening is at first central, but soon, by a process of unequal growth, becomes 

 carried towards the posterior end of the body. The pairs of stiff hairs in the 

 meantime considerably increase in number. The remains of the yolk cells 

 now disappear, and the enteric walls become more distinct. The alimentary 

 canal, which is at first simple in outline like that of a rhabdoccelous Turbel- 



