LECTURES ON EMBRYOLOGY. 



67 



is free through life; but all undergo similar changes 

 in early life. 



The Horse-Shoe Crab, though large, and in many 

 respects somewhat more complicated in its struc- 

 ture, belongs also to the Crustacea which have not 

 a calcareous, but a horny shell, and are called Ento 

 mostraca. 



From these facts, you may observe that Natural- 

 ists divide the Crustacea into two great groups; 

 those furnished with a shield, like the Crab and 

 the Lobster, called Malacostraca, and such as are 

 not thus protected,called Entomostraca, which have 

 only a horny envelope, and in which all the parts 

 are less diversified. 



I may mention more particularly one of these 

 Entomostraca (Plate XVIII, fig. F) a species of Ca- 

 lanus, which has a peculiarity of being phosphor- 

 escent, and of presenting a peculiar kind of phos- 

 phorescence which I am not aware has been ob- 

 served before. Here the nervous system, with the 

 eyes,is the shining part of the animal ; that nervous 

 system being not only phosphorescent, but the 

 substance of the nerves being of a highly red col- 

 or. The arrangement of the parts is precisely 

 the same as in the nervous system of the Crusta- 

 ceans in general, A close investigation of this 

 arrangement has shown me. that there can be no 

 mistake about it. 



[PLATE XXII EGGS OF PINNOTHERES.] 



The embryonic growth of Crustacea has been 

 extensively studied. We have had numerous mo- 

 nographic investigations upon that subject, which 

 were made by the most eminent of the Embryolo- 

 gists of our day. Rathke, in particular, has in- 

 vestigated that subject to a greater extent than any 

 one else. However, the earliest changes which the 

 egg undergoes, have not been so completely exam- 

 ined. Therefore,allow me to call your attention for 

 a few moments to the transformations of the eggs 

 of the little Parasitic Crab, the Pinnotheres Os- 

 triun^which is found in Oysters, and lives as nPar- 



asite between the gills of this animal. The whole 

 animal is so transparent that its growth and 

 changes can be very easily investigated. And 

 there we find eggs of various degrees of develop- 

 ment, some exceedingly minute, which consist of a 

 simplely vitelline membrane,with an absolute trans- 

 parent yolk, a small gerrninative vesicle and ager- 

 minative dot in the centre ; a few granules are no- 

 ticed in the yolk substance. Others will present 

 the same appearance in general structure, when 

 the germinative vesicle will be much larger, and 

 the germinative dot also much larger, being 

 swollen into a small vesicle. The same will be 

 universally observed in a series of changes, where 

 we notice that the germinative dot may groAY 

 much larger than it was before, and even form a 

 hollow vesicle within the germinative vesicle it- 

 self; the yolk granules having greatly increased 

 in quantity between the germinative vesicle and 

 the vitelline membrane. So that here it is perfect- 

 ly plain, that, the germinative dot can grow into a 

 hollow vesicle ; and from the condition of other 

 eggs, we may be satisfied that there is a period 

 when the germinative vesicle and the germinative 

 dot may disappear, to give rise to the formation of 

 another germinative vesicle containing more, nu- 

 merous granules; and that that vesicle may burst 

 again, and give rise to the formation of two germi- 

 native vesicles with their germinative dots, or we 

 may haye three germinative vesicles with their 

 germinative dots. And during this period of evo- 

 lution of cells within cells, there is an increase of 

 the mass of yolk taking place, an accumulation of 

 granules growing, by which that egg finally as- 

 sumes that degree of maturity! which precedes the 

 first formation of a germ. 



[PLATE III EGGS AND DEVELOPMENT OP 

 SHRIMPS ] 



I have traced these eggs up to the moment when 

 the yolk had become a mass of somewhat opaque, 

 though not very compact yolk, and the first rudi- 

 ments of an embryo were formed, as a disc on 

 one side of the egg, growing around it, and pre- 

 senting all the changes which hare already been 

 described by Rathke and Erdl, as occurring con- 

 stantly in the growth of Crustacea, and to which I 

 will now allude, referring to the species which he 

 has figured. 



The earliest condition of these germs in Palse- 

 mon, (Plate III. fig. A.) after the egg itself has nn- 



