UROCHOKDA. 



17 



The increase of the branchial clefts is somewhat complicated. Between 

 the two primitive clefts two new ones appear, and then a third appears 

 behind the last cleft. In the interval 

 between each branchial cleft is placed 

 a vascular branchial vessel (fig. 8 vi. 

 bb.). Soon a great number of clefts 

 become added in a row on each side 

 of the branchial sack. These clefts are 

 small ciliated openings placed trans- 

 versely with reference to the long axis 

 of the branchial sack, but only oc- 

 cupying a small part of the breadth TAIL 

 of each side. The intervals dorsal 

 and ventral to them are soon filled by 

 series of fresh rows of slits, separated 

 from each other by longitudinal bars. 

 Each side of the branchial sack be- 

 comes in this way perforated by a 

 number of small openings arranged in 



rows, and separated by transverse and FlG- 1L DIAGKAM OF A VEKY YOUNG 



longitudinal bars. The whole struc- ASCIDIAN. (From Lankester.) 

 ture forms the commencement of the 



branchial basketwork of the adult; the arrangement of which differs con- 

 siderably in structure and origin from the simple system of branchial clefts 

 of normal vertebrate types. At the junction of the transverse and longitu- 

 dinal bars papillae are formed projecting into the lumen of the branchial 

 sack. 



After the above changes are far advanced towards completion, 

 the openings of the two atrial sacks gradually approximate in the 

 dorsal line, and finally coalesce to form the single atrial opening 

 of the adult. The two atrial cavities at the same time coalesce 

 dorsally to form a single cavity, which is continuous round the 

 branchial sack, except along the ventral line where the endostyle is 

 present. The atrial cavity, from its mode of origin as a pair of epi- 

 blastic involutions 1 , is clearly a structure of the same nature as the 

 branchial or atrial cavity of Amphioxus ; and has nothing whatever 

 to do with the true body cavity. 



It has already been stated that the anus opens into the original 

 left atrial cavity ; when the two cavities coalesce the anus opens into 

 the atrial cavity in the median dorsal line. 



Two of the most obscure points in the development are the origin 

 of the mesoblast in the trunk, and of the body cavity. Of the former 

 subject we know next to nothing, though it seems that the cells 



1 In the asexually produced buds of Ascidians the atrial cavity appears, with the ex- 

 reption of the external opening, to be formed from the primitive branchial sack. In 

 the buds of Pyrosoma however it arises independently. These peculiarities in the buds 

 cannot weigh against the embryonic evidence that the atrial cavity arises from in- 

 volutions of the epiblast, and they may perhaps be partially explained by the fact that 

 in the formation of the visceral clefts outgrowths of the branchial sack meet the atrial 

 involutions. 



B. E. II. 



2 



