90 iudpokt — 1877. 



a nearly isolated sphere, the first of tli9 direction-corpuscles, while a second ra- 

 diating system arises in an unknown manner within the egg, and, approaching the 

 still included half of the first spindle, forms with it a second spindle, which, by a 

 similar process, gives rise to the second direction-corpuscle. 



A third radiating system then appears, similar in most respects to the preceding 

 one, but having in its centre a small nucleus, which, becoming vacuolated, ulti- 

 mately combines with the nucleus of the included end of the last spindle, derived 

 from the nuclear plate, and forms with it the segmentation-nucleus. This third 

 radiating centre Hertwig believes, from its close similarity to the second or Samen- 

 kern of the Sea-urchin egg, to be the spermatic nucleus. 



From these and other confirmatory observations, which time forbids me to refer 

 to, we are warranted in drawing the conclusion that the germinal vesicle is not 

 entirely expelled, but that it undergoes a transformation, mixing in part with the 

 neighbouring yolk, and, having usually parted with a portion of its substance in the 

 form of the Richtungsblaschen, it supplies one essential element to the formation of 

 the segmentation-nucleus for further development. 



On the third line of inquiry, namely, as to the nature and origin of the direction- 

 corpuscles, there has been hitherto no accurate concordance among observers. 

 .Robin* describes these as derived from the homogeneous yolk-substance ; Biitschlif, 

 Oellacherf , and others consider them to be formed from the extruded germinal 

 vesicle, or at least to contain a part of that structure. Oscar Hertwig §, in this 

 connexion, has attempted to distinguish two kinds of bodies emitted from ova ; to 

 the first he limits the name IHchtungsbliisehm, and these he characterizes as always 

 arising by a process of budding, to which the division of the nucleus is a necessary 

 preliminary. Such true direction-corpuscles he considers as representing a kind of 

 suppressed parthenogenesis, a theory which is borne out by the observations made 

 by Richard Hertwig || on the infusorium Spirochora gejnmipara , wherein he shows 

 that similar spindles are formed before fission takes place in that form. Stras- 

 burger^T, on the other hand, who has discovered similar bodies in the ovules of coni- 

 fers, supposes that by these corpuscles certain elements are got rid of, which, by 

 their presence, prevent development in the egg. 



From the true MichtunffsblascJien Hertwig distinguishes the simple protoplasmic 

 matter produced from some eggs, not by any process of budding, and apparently 

 with no precedent spindle-stage. These he calls, after Fol, " excretion-splieres." 



On this subject we require much additional information before we can feel war- 

 ranted in drawing any inferences or conclusions as to the true nature of these re- 

 markable bodies. 



Among the observations on special parts of later stages of embryonic growth, I 

 shall ask your attention specially to two sets of recent researches : — 1st, those on 

 the primitive groove of the fertilized egg ; and 2nd, those on the embryology of 

 limbs. 



As most of my hearers are aware, Prof. Dursy ** first described, several years 

 ago, that in one of the earliest stages of the developing embryo there is formed a 

 primitive groove, which, having appeared and lasted for a short time, finally 

 vanishes, being pushed out of existence by the medullary groove, the first trace of 

 origin of the spinal cord. This arrangement, singular from its transitoriness, has 

 naturally attracted much attention, and various hypotheses have been advanced 

 to account for its existence ; some, like the veteran embryologist Kolliker, have 

 expressed their belief that this groove is but a continuation of the medullary groove, 

 with which, prior to Dursy's discrimination, it had always been confounded, and 

 that its margins, the primitive folds, are simple extensions of the medullary folds ; 

 but such a view fails to account for its obvious distinctness, for its frequent want 

 of symmetrytt, for the evident fusion beneath it of the embryonic layers, epi- and 

 mesoblast, and probably also, at its hinder end, of the hypoblast (though Kolliker 



* Journal de la Physiologie, 1862. 1 Op. eit. 



J Journal de la Physiologie, 1802. § Morph. Jahrb. Bd. iii. p. 70. 



|| Jenaische Zeitscbrift, 1877, p. 149. «|[ Zellbildung, p. 294. 



** Der primitive Streif des Hiihnchens. 



tt Goette, Archiv fiir niikrosk. Auat. vol. x. p. 145. 



