THE DEVELOPMENT OF ECHINOCARDl UM CORDATUM. 475 
first rudiments of the larval skeleton have appeared [calc., 
fig. 2). They are in the form of two calcareous stars 
situated to the right and left of the middle line. In each 
‘^star^^ one arm grows outwards and impinges on the ecto- 
derm and so causes the formation of one of the first pair of 
arms of the larva. In Echinocardium this outwardly 
directed rod is accompanied by another rod parallel to it^ 
which arises as a branch from another arm of the star. Tliese 
parallel rods are connected at intervals by cross-bars, and are 
conveniently regarded as one compound latticed rod which 
receives the name of post-oral rod (p. o.r., fig. 3), and which 
forms the skeleton of the post - oral arm. The post-oral arms 
which are thus the first of the larval arms to be formed are often 
termed anal arms by German writers on Echinoid develop- 
ment (p. 0. a., fig. 3). From the apex of the archenteron 
mesenchyme cells continue to be given off. These cells, 
which are termed secondary mesenchyme {s. mes., fig. 2), 
form the wandering cells which traverse the blastocele or 
primary body-cavity of the larva and also form the delicate and 
sparse connective tissue which is found in this cavity later. 
We have seen that the post-oral rods arise, each from an out- 
wardly-directed branch of one of the calcareous stars. The 
other branches of the calcareous stars grow in length, and one 
which extends downwards towards the lower pole of the larva 
is termed the body -rod; another which extends hoi'izoutally 
inwards towards the middle line along the posterior surface of 
the larva is termed the ho rizon tal rod. A third bi-anch of 
each star which extends upwards towards the upper pole of the 
larva is termed the antero-lateral rod. At the lower pole 
of the larva there is to be seen a mass of mesenchyme cells 
wedged in between the lower ends of the two body-rods. In 
this mass there can be detected a minute calcareous star 
{ab. calc., fig. 4) which is the rudiment of the skeleton of the 
aboral spi k e. 
In the meantime a bilobed outgrowth appears at the apex of 
the archenteron; this is the rudiment of the coelom. It soon 
becomes cut off and divided into right and left sacs {coe., fig. 4), 
