OCCURRING IN THE NEIGHBOURHOOD OF PLYMOUTH. 49 



witliout having any great importance to the latter ; or we may suppose 

 that as the tissues of the adult are oily, it is necessary that the tissues 

 of the embi'yo should be supplied with abundance of oil in order to 

 develop normally. But perhaps the truth lies in the union of these 

 two suppositions, that the excess of oil in the tissues of the parents 

 extends into the ovum, and during the development of the latter 

 supplies the embryo with an abundance of fat which is necessary to 

 its constitution. But none of these hypotheses explain why in 

 many cases ova provided with oil-globules have a greater specific 

 gravity than those that are witliout them ; a difference which must 

 depend on a greater density of the protoplasm and of the yolk. 



The Development op the Vascular System and Ccelom in Pelagic 



Ova of Teleostei. 



In a great many pelagic Teleostean embryos at the time of 

 hatching the heart consists of a tube which opens posteriorly out 

 of a wide space between the yolk, or more accurately between the 

 surface of the periblast and the wall of the yolk-sac, which wall 

 consists solely of a layer of epiblastic cells. The heart itself is 

 surrounded by another cavity which is separated from the space first 

 mentioned by a thin membrane, which passes on the one hand into 

 the lips of the posterior aperture of the heart, and on the other into 

 the body wall ventrally, into the tissue beneath the pharynx dorsally. 

 The space out of which the heart opens contains blood, i. e. a 

 colourless fluid containing at first colourless corpuscles, which at a 

 later stage become red corpuscles. The blood is carried by the 

 pulsations of the heart out of the space round the yolk into the 

 cavity of the heart. 



The cavity round the pei'iblast, which communicates with the 

 heart, exists at an earlier stage, before hatching, as a space between 

 the epiblast of the anterior part of the yolk-sac and the periblast. 

 And this is the same space which exists at a still earlier stage before 

 the yolk has been enveloped by the blastoderm, between the epi- 

 blast and the periblast in the central part of the blastoderm, that is, 

 over all the region of the latter which is not occupied by the 

 embryonic rudiment and the embryonic ring. This space, in fact, 

 is the earliest to appear in the ovum, and is nothing more or less 

 than the segmentation cavity. 



To consider now the space which surrounds the heart, and which 

 is entirely separated from the space which communicates with the 

 heart. This cavity round the heart is simply a portion of the true 

 body-cavity or coelom. The heart with this cavity develops shortly 



VOL. II, NO. I. 4 



