[J02] 



OVUM. 



tears of ike vesicle,. the more immediately 

 germinal part of the egg is formed from the 

 mixture of the two. However this may be, it 

 seems not improbable, from the observations 

 now referred to, that the spermatozoa are 

 conveyed directly to the germinal part of 

 the egg by the funnel of the micropyle. I 

 shall afterwards have to state the more 

 numerous instances in which, following its 

 first discovery by J. Miiller in the Holo- 

 thuria, a micropyle has been detected in the 

 ova of Invertebrate animals ; and I may at- 

 tempt to show the great importance of this 

 aperture in connection with fecundation in ova 

 with thick external coverings to which the 

 spermatic substance does not gain access till 

 the later periods of their formation. The ac- 

 companying figures of the micropyle in the 

 Stickleback will give a sufficiently clear view 

 of this remarkable structure. At present 

 it may be permitted to remark that, if 

 we consider the size of this aperture, and 

 the ease with which it may be found in 

 the ova of fishes by an observer whose at- 

 tention has been called to its existence, to- 

 gether with the fact of its having been so long 

 overlooked previously, there is much ground 

 for caution as to negative statements as to 

 the existence of a similar aperture in the ova 

 of other animals. I have already made al- 

 lusion to this subject in the previous sections, 

 in which I have stated that Dr. Ransom has 

 expressed to me his firm conviction, founded on 

 observations, that the micropyle exists also in 

 the ova of Batrachia. At the same time it is 

 quite probable that such an aperture may only 

 exist or be required for the admission of the 

 spermatozoa when fecundation is of late oc- 

 currence, and when the covering membrane 

 of the ovum is so dense as to resist the pene- 

 tration of the spermatozoa through its solid 

 substance. 



It is right also to mention that the exist- 

 ence of this aperture, or rather the funnel lead- 

 ing to it, did not entirely escape the observa- 

 tion of preceding physiologists. The accurate 

 Von Baer, in his work on the development of 

 Fishes*, has described in the Bream (Cy- 

 prinus blicca) a funnel-shaped depression of 

 the external membrane, which reached nearly 

 to the surface of the germ ; and he ob- 

 served that this funnel was effaced as soon 

 as the imbibition of water took place. He 

 considered this aperture as most probably 

 owing to the escape of the germinal vesicle 

 from the surface of the yolk and through the 

 coverings of the ovum, "in the same manner 

 as he had described in the frogf, and did not 

 therefore conceive it to serve any immediate 

 purpose in connection with the introduction 

 of the spermatozoa. Dr. Ransom has ob- 

 served that the effacement of the funnel which 

 he had seen in the Stickleback is not inva- 

 riably the consequence of fecundation in the 

 Fish's ovum; for in the salmon and trout 



* Entwickelungsgeschichte der Fische, Leipzig 1 , 

 1835, p. 9. figs. 1. and 2. 

 t De Ovi Mammal. &c., pi. xxv. 



Fig. 69*. 



Development of the ova of Gasterosteus. 



A. B. c. D. Four ova of the Stickleback in the 

 earlier stages of their development within their 

 ovisacs. 



In that figured at A, which is the earliest, ^" 

 in diam., the germinal vesicle placed near the cen- 

 tre has scarcely any perceptible membrane or wall, 

 hut resembles a gelatinous mass in which the small 

 number of maculae are developed : there is as yet no 

 yolk, but only a slightly turbid fluid substance 

 filling the space between the ovisac and the ger- 

 minal vesicle: delicate epithelial cells project from 

 the inner surface of the ovisac. 



In B. ^y the maculse have increased in number, 

 the germinal vesicle, as well as all the other parts, has 

 increased in size, the fine granules of the yolk sub- 

 stance have begun to be deposited towards the 

 periphery, but there is as yet no vitelline mem- 

 brane. The wall of the ovisac is now more distinct, 

 and besides the internal cells, there are seen on the 

 exterior the nuclei of external flattened cells. 



In c. -fig" the maculai have become more numerous 

 and distinct; the yolk granules are more opaque 

 and in greater quantity, and the mass of the j-olk 

 more circumscribed, a clear space now intervening 

 between it and the wall of the ovisac. 



