16 



PROF. AGASSIZ'S 



upon the upper surface with little holes through 

 which the eggs escape. The ovary itself is a gra- 

 nular organ, of which several figures in various 

 stages of developement are here seen. 



'PLATE IX OVARIES OF 



Such is the structure of the Echinoderms, the 

 stomach forming a simple cavity, without any oth- 

 er outlet except the mouth, In some the alimen- 

 tary tube is more complicated. In the Echini or 

 Sea-Urchins [Plate VI] there is an alimentary tube 

 forming several evolutions, and opening upwards. 

 The ovaries form more peculiar masses than in the 

 star-fishes. The mouth is also protected in most 

 Echini by a complicated set of jaws and teeth. In 

 Holothurise the whole system of organs assumes a 

 more bipartite arrangement. 



In the process which gives rise to the formation 

 of new individuals, the first step consists in the ac- 

 cumulation of more or less consistent matter of a 

 somewhat opaque or yellowish appearance, and of 

 a granulated texture which divides soon into small 

 spherical masses. This takes place in the ovary. 

 This mass, at first homogeneous, assumes soon the 

 aspect of little bunches, which soon grow more and 

 more isolated, and then assume around them a pe- 

 culiar membrane, and there are eggs. Eggs in their 

 simplest condition are microscopical spheres of a 

 homogeneous mass, called yolk, and surrounded 

 by a simple membrane, called the yolk membrane. 

 (Plate IX C.) However different in its aspect in 

 different animals, this mass is called yolk, through- 

 out the animal kingdom, from the fact that this 

 name has been applied to the part which corres- 

 ponds to this structure in the hen's esg. 



The primitive egg is always microscopical, and 

 its contents homogeneous ; but this substance soon 

 becomes granular. It is so small as to escape the 

 observation of the naked eye. And there is anoth- 

 er little sphere formed within, which is called the 

 Kerminative vesicle, containing another little ves- 

 cicle, which is called the germinative dot. [Plate 

 IX fig. D.] Under a powerful microscope the gran- 

 ules of the yolk itself appear also like little cells. 

 There are little spherical masses, and they contain 

 even in their turn other little dots. 



Plate IX shows the various degrees of the growth 

 of such eggs, of which there are more or less de- 

 veloped ones in the same ovary ; assuming first 

 their regular form [Fig. AJ, and then a transparent 

 space appearing in the interior [Fig.B] ; next the 



germinative vescicle becomes more distinct fii^' 

 C], and [Fig. DJ the germinative dot is now dis- 

 tinctly seen. The whole mass of yolk, which ha& 

 grown considerably, consists here of cells, which 

 have been formed by the expansion of its granules. 

 Through this growth of cells within cells, and of 

 granules growing into cells, there is finally a germ 

 formed, That which we call yolk in the beginning, 

 is finally a spherical germ, which will escape from- 

 its envelope. We have here [Fig. E} the ovary of 

 a star-fish, from which some germs have escaped, 

 and here is the figure of such a germ already 

 hatched, highly magnified [Fig. F]. The ovary of 

 sea urchins, have all the same structure, and 

 vary only in their size and proportions. Now a 

 curious observation which I have had an opportuni- 

 ty to make, is, that the eggs after they are laid are 

 taken up by the star-fish, and kept between its 

 tubes, below the mouth. The star-fish bends itself 

 around them, surrounds the eggs with its suckers, 

 and moves about with them. When the eggs had 

 been removed to some distance from the animal, it 

 went towards them and took them up again, and 

 moved off with them, showing that these animals^ 

 so low in structure, and apparently deprived of all 

 instinct, really have so much instinct as to watch 

 over their young. 



Now these eggs which are thus kept there, and 

 protected by the mother, will escape. These germs- 

 I have been able to trace from the lowest possible 

 condition, where they resemble ovarian eggs. At 

 no epoch did I see this new born animal living free y 

 and swimming like Infusoria, as is said to be the 

 case by Sars. 



Soon, however, the external crust of the germ- 

 becomes more transparent, consisting of somewhat- 

 looser and larger granules, and the internal mass 

 assumes a color a little darker, so that two layers- 

 are distinct, between which there is another one, 

 which becomes also gradually more and more dis- 

 tinct. On one side of the germ there is now a pro- 

 tuberance forming, and the prominent portion 

 separates more ancUmore from the spherical mass ? 

 [Plate IX, F] the difference in substance of Ms 

 layers growing more and more distinct. The promi- 

 nent portion, which is the lower part of the little 

 animal, becomes more and more elongated and as- 

 sumes more and more the form of a peduncle. 

 Often there are several grouped together, and at- 

 tached by this appendage to the empty egg casesl; 

 they would even form bunches remaining thus 

 attached till they are far advanced in their growth. 

 At this period, however, there is not yet any or*' 

 gan formed as you will notice on comparing Fig. 

 F of Plate IX. with those of Plate IV. p. 13. Onljr 

 changes of substance have taken place. But now 

 we begin to see little swellings in five points oc? 

 the sides ; the spherical portion of the germ ha 

 also grown considerably, and has been flattened by 

 lateral dilatation. 



The little animal has grown to a more hemis- 

 pherical shape ; and from that time there is an up- 



