AND THE DEVELOPMENT OF THE MYRIAPODA. 105 



condition. They have always exhibited the same yellow colour, with only a slight 

 difference of opacity. At a later stage of development the whole appear to be in- 

 closed in a distinct membrane, the memhrana vitelli (fig. ll.rf), and even at this 

 early period (fig. 8.) the membrane may be regarded as in the course of formation, 

 as seems to be indicated by the fact, that the cells always cohere together when the 

 specimen has been placed in spirits of wine, and afterwards allowed to remain for 

 some time in water. In these cases the ovisac becomes distended by the imbibition 

 of fluid, as is proved by the existence of a clear transparent space between the interior 

 of the ovisac (figs. 7 to W.d) and the yelk (e), the cells of which do not separate, but 

 together retain the form of a single mass unaltered, even when pressed in different 

 directions between plates of talc beneath the microscope. When the ova are a little 

 further advanced (fig. 9.), the same yellow-coloured graniform cells compose the 

 yelks ; but they are a little darker in colour, larger, and more distinctly exhibit a 

 granular structure in their interior : they closely resemble in form and structure the 

 vesicles of the yelk of the higher animals, as described in the excellent researches 

 of Dr. Martin Barry*. The membrane of the yelk (d) at this period is more strongly 

 marked, and exhibits a distinct outline, when treated as above described ; and the 

 germinal vesicle is much more apparent (a, b). At this stage of the ovum, the outline 

 of the vesicle is more distinct than in the previous stages, and this body is slightly 

 larger. The macula (a) is of a perfectly globular form, and is apparently covered 

 by a separate membrane-}-. It is very distinct, and is formed by an aggregation of 

 minute cells, or vesicles, surrounded by fluid. In a more advanced stage of deve- 

 lopment the vesicle is surrounded at a little distance by an outer ring, which exists in 

 all the ova in the succeeding stages, but is not seen in the first stages of the ovum. 



When a perfect egg that has passed into the oviduct, and is ready to be deposited, 

 has been placed in spirits of wine, and is afterwards examined beneath the microscope, 

 there are seen on the outside of its opake shell one or more large spots (Plate IV. 

 fig. 5. b. c c), which appear to be occasioned by some circumstance connected with 

 the formation of the shell in the oviduct or ovary. These spots are large and oval, 

 are formed by concentric rings, and appear to be the result of incomplete depositions 

 of the different layers of material of which the shell is composed. In the vicinity 

 of the spots within the egg I have usually found some aggregated oil-globules. The 

 yelk is of a light yellow colour, and occupies nearly the whole of the interior, there 

 being only a very small space around it, and a very slight quantity of albumen. The 

 form of the yelk within the shell, when the egg has remained for some time in spirits 

 of wine, is irregular. At one end it becomes obtuse, and rounded ; at the other 

 incurved, and a little pointed. On one surface it is large and convex, and on the 

 opposite concave or excavated. In the middle of the concavity is the transparent 

 globular vesicle, the proper germ vesicle, considerably enlarged, and presenting the 



* Philosophical Transactions, 1838, 1839, 1840. 



t See Wagner's Physiology, translated by Dr. Willis, Part I. 1841, p. 41, note 48. 



