70 



PROF. AGASSIZ S 



LECTURE IX 



More than once I have alluded to the uniformity 

 of structure of the egg, in its primitive condition, 

 in all animals ; thus showing that there is a com- 

 mon starting point for their growth, throughout 

 the various classes of the animal kingdom, I 

 shall now illustrate more fully the physiolog- 

 ical process by which the egg, when matured, 

 gives rise to the formation of a germ. I do not 

 intend this evening to enter into more details than 

 I have already given, upon the formation of the 

 egg itself, but to illustrate the process by which 

 the egg gives rise to a germ. This process has 

 been traced in all classes of the animal kingdom ; 

 and it is found to consist of a very complicated se- 

 ries of changes taking place in the substance of the 

 yolk, when it has reached a certain degree of ma- 

 turity. 



The condition, therefore, the first essential and 

 constant condition for the formation of a germ, is 

 the previous formation of an egg, and its being 

 matured to a certain degree. The size, the degree 

 of maturity, and changes which the egg itself un- 

 dergoes before the germs are formed, vary in dif 

 ferent classes. I will not allude to that point at 

 all, but only take now the germ as it is forming 

 within the egg, when the yolk has grown to a cer- 

 tain size. 



[PLATE XXIV EGGS OP 



I cannot, however, omit mentioning a very curi- 

 ous mode of ovulation which is noticed in some 

 Worms. When, some months befo:e the laying o ( 

 the eggs, we observe the ovary of the Nemertes, 

 we see in their interior, oblong, bottle-shaped 

 pouches forming, which fill with yolk substance, 

 that gives rise to the eggs. When these bottles 

 have attained their whole development, that is to 

 say, when they are completely filled with yolk 

 substance, a new process is introduced in them. 

 The substance groups itself around several centres, 

 and forms a series of little spheres, whose number 

 varies. These are the eggs ; eggs which soon 

 have a germinative vesicle, and within it, a germi- 

 nal ive dot characteristic of the eggs in general. 

 When this second progress is terminated, the bot- 

 tles are laid, under the shape of a chain, and the 



eggs are thus contained in a transparent sub- 

 stance of shapeless appearance. 



After the laying of the eggs, another series of 

 transformations is produced, as we shall see pres- 

 ently. Almost the same changes occur in the 

 Malacobdella, which is a Parasitic Worm found in 

 the Clam. There, also, we have observed yolk 

 bottles, as also the successive formation of the 

 eggs. Here there is no ovary proper ; we have 

 found the bottles distributed in the whole body 

 around the intestinal canal. Some contained only 

 one egg, and some not yet condensed yolk sub- 

 stance ; others contained two eggs ; others three, 

 four, and even a greater number were formed, 

 until the whole yolk was exhausted. 



Plate XXV represents some of these phases. 

 In the Planarise the mode of formation of the eggs 

 is the same, except the bottles. 



[PLATE XXV EGGS OF MALACOBDELLA ] 



Let us return to the egg, when it is about enter- 

 ing another series of changes. In Piates XXIV 

 and XXV, we have eggs of different animals, 

 in which the process of the formation of the 

 germ is represented up to a certain degree of 

 its growth. The primitive egg consists, as you re- 

 member of a vitelline membrane containing yolk, 

 and within this yolk a germinative vesicle,and with- 

 in that a germinative dot, as shown in Plate XXIV, 

 A, B. The yolk becomes gradually more and 

 more condensed, thickened, and more and more 

 opaque; and at that epoch, the germinative vesi- 

 cle generally disappears ; the germinative dot dis- 

 appears also, and new changes begin to take place 

 within the yolk. 



It has been questioned, whether the germinative 

 vesicle and the germinative dot precede, or follow 

 the formation of the yolk substance. There are 

 examples of ovarian eggs in which this vesicle and 

 this dot are very distinct, as also the yolk mem- 

 brane, at the time when the vitellus is yet very thin 

 and transparent in the sphere of the egg. We have 

 seen this vitellus increase and fill up the whole 



