Miscellaneous, 283 



forty-two hours, according to the temperature, and various other 

 circumstances. From thirteen to fifteen minutes after the impreg- 

 nation, the vitelline mass is seen most commonly to vibrate and be- 

 come animated with a more or less rapid rotatory motion. From 

 the fourth to the sixth hour the vitellus begins to divide, and the 

 segments which result from this division become more and more 

 transparent. Then a number of little globules are produced on the 

 surface, from the large globules, subsequently surrounding them 

 completely, and thus constituting a pretty thick layer. When this 

 layer of globules, which is the rudiment of the teguraentary enve- 

 lope, is extended to the whole of the vitelline surface, the embryo 

 has nearly acquired the form under which it will issue from the egg. 

 The vitelline membrane, very distinct during the first period of the 

 division, has completely disappeared, and the albumen, at first opa- 

 line, has become as transparent as sea- water. Soon after, the tegu- 

 mentary surface of the embryo is covered with filiform appendages, 

 of extreme tenuity. Generally towards the twenty-fourth hour, 

 but sometimes a little later, the embryo agitates its appendages with 

 a great velocity, which have acquired sufficient force to serve it as 

 locomotive organs. The animal then soon divests itself of the tes- 

 taceous membrane of the e^^. 



At the moment of hatching, the larva of the Echinus has a form 

 very analogous to that of the Medusae and the Radiata in general. 

 Its body is rounded like that of the adult animal, presenting simply, 

 at one point, a slight concavity, in the centre of which is the 

 outline of the mouth. That portion may be distinguished whose 

 degree of development is more advanced than other parts of the 

 body, by the name of oral pole. By the aid of its filiform appen- 

 dages the larva moves with tolerable facility, and almost always re- 

 volving upon itself. In the sixth or eighth day the form of the 

 animal is modified ; half its body, that where the anus is situated, 

 and which may be termed the anal pole, is a little elongated. The 

 surface of the exterior envelope has become more compact and 

 transparent ; the large globules which were in the centre of the body 

 have disappeared. We then observe the first rudiments of the in- 

 testinal canal, in which a short oesophagus is distinguished, a sto- 

 mach having the form of a large ampulla, and a very short intestine. 

 About the twelfth or the fifteenth day, the body of the larva has be- 

 come completely pyriform ; the circumference of the anus presents 

 little discs forming a sort of small rosette, and deep circular lines are 

 seen on the portion of the tegument comprised between the two poles; 

 the dimension of the oral pole has considerably increased, and we 

 now perceive, around the mouth, appendages analogous to labial 

 tentacula. 



Arrived at this stage of development, that is to say, towards the 

 sixteenth to the eighteenth day, the larva of the Echinus, which has 

 lost all its agility, attaches itself, by the anal pole, to the body near 

 which it has rested ; and a cylindrical pedicle of tolerable size, and 

 once and a half as long again as the diameter of the body, is deve- 

 loped very rapidly. Thus fixed on a flexible stem, the young ani- 



20* 



