TRANSACTIONS OK THE SECTIONS. 8'J 



masses are extruded at one pole of the vitelline mass. To these the name polar 

 bodies, or RicldtmgsbUischm, direction-vesicles, has been given ; and it has been 

 conjectured that the appearance of these corpuscles is in some way related to the 

 vanishing of the germinal vesicle. The three problems which this primary stage of 

 development presents for solution are :— 1st, AVhat is the method whereby the sti- 

 mulus to development directly operates on the egg? 2nd, What becomes of the 

 germinal vesicle P and 3rd, In what manner and from what source the direction- 

 corpuscles arise, and what function do they serve in the animal economy? 



In relation to the first inquiry, the researches of E. van Beneden * and of many 

 others show that on the surface of the ovum, in the mammal, spermatic nuclei are 

 observable before development ; and the careful researches of Oscar Hertwigt, and, 

 after him, of Fol show that in the purple Sea-urchin (Stronr/i/locentroUis liridus) 

 the nucleus or head of one of these traverses the vitelline membrane of each egg, 

 enters the yolk, and therein travels until it reaches the nucleus of the egg ; where- 

 upon the two, by fusion, form the segmentation-nucleus, which then begins to 

 cleave and to set tip the conditions which we formerly looked upon as primary in 

 the developing egg. In certain Leeches also, Ilertwig f has shown that the entrance 

 of the spermatozoa takes place in the same way; and in the egg of the Frog the 

 track of the entering nucleus has been even more easily observed, being indicated 

 by a streak of clear protoplasm in the midst of the dark pigment of the egg. We 

 may correlate with this Auerbach's§ observation, that in certain Nematodes nuclei 

 become approximated in the egg, and thereby originate the segmentation-nucleus; 

 and while the exact nature of the burrowing nucleus has been questioned by 

 Butschli, yet Hertwig's discovery of a tailed spermatic nucleus burrowing into the 

 egg in Strongyhccntrotus, and a similar instance quoted by him from Hensen, 

 together with the absence of any probability or reason to the contrary, seem to 

 render it certain that the ingoing nucleus is cither an unaltered spermatozoon, or 

 (though this is less probable) a compound nucleus of vitelline and spermatozoal 

 elements, as E. van Beneden has surmised. 



In the second place, as to the fate of the germinal vesicle, the results of observa- 

 tions have been various, although reconcilable. Thus, Oellacher || describes that 

 in the eggs of the Trout the germinal vesicle is emitted, having first reached the 

 surface ; Goette f finds that in the bombardier frog (Bombinator) the germinal 

 vesicle seems to vanish without visibly approaching the periphery; while Butschli 

 finds that in Nematodes the vesicle is extruded as a direction-corpuscle, and Van 

 Beneden states -that at least some of the materials of the vesicle are extruded ; 

 Ilertwig, who has studied these transformations in Leeches with great care, describes 

 that at the beginning of development the nucleolus or germinal spot first appears 

 amoeboid, then divides, while simultaneously the wall of the germinal vesicle thins 

 and vanishes, its place being marked by a clear non-granular space, within which a 

 spindle-like body forms, consisting of a central plate of granules, formed from the 

 broken-up nucleus, and rays of protoplasm disposed in two tapering cones fore and 

 aft from this central " nuclear plate," as Strasburger has called it, the whole being 

 a somewhat closterimn-like or caraway-seed-like figure, at each of whose extremi- 

 ties there is a clear space surrounded by an aureole of star-like rays in the proto- 

 plasm of the egg. The material of this spindle, or carylolytic figure, as it is called, 

 is derived from the germinal spot and from some of the original protoplasmic con- 

 tents of the now indistinguishable germinal vesicle. Approaching the surface, one 

 of the points of the spindle makes a hillock or projection on the periphery of the 

 egg, then the nuclear plate divides transversely, one layer being displaced outwards 

 into the papilla, while the other remains within the circumference of the egg- 

 sphere. 



The papilla then becomes constricted off from the surface of the egg, and forms 



* Bulletin de l'Acad. royale de Belgique, 2 e serie, t. xL (1875). 



t Morphologische Jahrbuch, Bd. ii. iii. 



\ Morphologisehe Jahrbuch. 



§ Organologische Studien, Heft ii., 1874. 



|| Arcbiv fur mikrosk. Anat. Band viii. 



«[ Entwickeluugsgeschichtc der Unke. 



