THE BODY CAVITY. 



515 



Ill all other cases they are formed in a region which appears to belong 

 to the epiblastic region of the cloaca ; and from my observations on Elas- 

 mobranchs it may be certainly concluded that they are formed thera 

 in this group. They may appear as perforations (1) at the apices of 

 papilliform prolongations of the body cavity, or (2) at the ends of cloacal 

 pits directed from the exterior towards the body cavity, or (3) as simple 

 slit-like openings. 



Considering the difference in development between the abdominal pores 

 of most types, and those of the Cyclostomata, it is open to doubt whether 

 these two types of pores are strictly homologous. 



In the Cyclostomata they serve for the passage outwards of the genera- 

 tive products, and they also have this function in some of the few Teleostei 

 in which they are found ; and Giegvnbaur and Bridge hold that the primi- 

 tive mode of exit of the generative products, prior to the development 

 of the Miillerian ducts, './as probably by means of these pores. I have 

 elsewhere suggested that the abdominal pores 

 are pet haps remnants of the openings of 

 sea-mental tubes ; there does not however 

 appear to be any definite evidence in favour 

 of this view, and it is more probable that 

 they may have arisen as simple perforations 

 of the body wall. 



Pericardial cavity, pleural cavities, 

 and diaphragm. In all Vertebrata the 

 heart is at first placed in the body cavity 

 (fig. 353 A), but the part of the body cavity 

 containing it afterwards becomes sepa- 

 rated as a distinct cavity known as the 

 p e v i c a r d i a 1 c a v i t y. i 11 Elasmobranchii, 

 Acipenser, etc. a passage is however left 

 between the pericardial cavity and the 

 body cavity; and in the Lamprey a sepa- 

 ration between the two cavities does not 

 occur during the AmmocoBte stage. 



In Elasmobranchii the pericardial 

 cavity becomes established as a distinct 

 space in front of the body cavity in the pericai . aial cavi t y by a horizontal 



following Way. When the two ducttlS septum in which runs the ductus 



Cuvieri, leading- transversely from the Cuvieri ; on the left side is seen 



, r 1 1 a the narrow passage which remains 



sinus venosus to the cardinal veins, be- connectlnR the two cavities. 



come developed, a horizontal septum, sp ^ Sllillill cana i ; w. white 

 shewn on the right side in fig. 352, is matter of spinal cord; pr. corn- 

 formed to Support them, Stretching across missure connecting the posterior 

 rJ . ' .. nerve-roots; en. notocnoiu, x. 

 from th3 splanchnic to the somatic 8 ub-notochordal rod; ao. aorta; 

 of the body cavity, and dividing the body sv. sinus venosus; cnv. cardinal 

 cavity (fig. 352) in this part into (1) a 

 dorsal section formed of a right and left 



ht 



FIG. 35'2. SECTION THROUGH 

 THE TIIUNK OF A ScYLLIUJI EMBRYO 

 SLIGHTLY YOUNGER THAN 28 F. 



The figure shews the separa- 

 tion of the body cavity from the 



ven ; ht. heart ; pp. body cavity; 

 pc. pericardial cavity ; <xs. solid 

 oesophagus; /.liver; mp. muscle- 

 division constituting the true body cavity plate. 



33?, 



