ON OUR PRESENT KNOWLEDGE OF THE CRUSTACEA. 207 



spot that is considered to be an imperfect organ of vision. The oph- 

 thalmic spot is visible at this period in the embryo. (PI. VII., fig. 5.) 



If we follow this examination throngh succeeding periods, we find the 

 progression of the development of the embryo to be distinct and con- 

 tinuous, and the changes important and reliable.- The small ophthalmic 

 spot is present and the two central lobes are still in apposition, but have 

 become more elongated. The first (a) of the lateral lobes has enlarged and 

 become more massive and consolidated in structure. The second and 

 third (b, c) have increased very considerably in length and lost the lobe- 

 like appearance, putting on that of more extended appendages ; whereas 

 those of the three succeeding pairs of lobes still retain their simple 

 lobe-like character. (PI. VII., fig. 5.) 



The several parts are now becoming very distinguishable in their 

 relation to the rest of the animal, and it is interesting as well as 

 instructive to examine the nature of the structure in detail. 



The first or most anterior pair of lobes, a, meet together at the anterior 

 extremity, at the union of which the ocular spot is visible, while they 

 are separated at the opposite by the intervening central lobe which we 

 have already determined to be the labrum (lb.) The entire mass differs from 

 the other portions of the embryo by being of an opaline and less trans- 

 parent appearance. It is formed by an aggregation of exceedingly minute 

 cells that appear to cohere closely together ; these lobes appear to be con- 

 tinuous with a great central mass that extends from one extremity of the 

 animal to the other. Soon we perceive some pigment cells forming a small, 

 dark, irregular stripe deep within the anterior lobes, a, and by its arrange- 

 ment apparently separating off a portion of the great opaline mass. 

 (PI. VII., fig. 5.) 



This stripe of pigment is the early or incipient condition of the great 

 black cornea that is so conspicuous an object in all young Crustacea. 

 At the same period, near the opposite extremity of the ovum a small 

 and irregular pulsation may be observed. This is the position of the 

 future heart. At first the pulsation is very slow, feeble, and irregular ; 

 a small corpuscular body may be seen jerked forward and backward within 

 a small sacular space or hollow, after unequal intervals of rest. After a 

 time a solitary corpuscle is seen to burst through an opening in the walls 

 of the sac. This at distant intervals is repeated, and after a time more 

 frequently, until in a day or two the throb of the sac becomes more con- 

 stant, the presence of the corpuscles more numerous, and the flow of 

 them increasingly more regular and continuous. 



The vitellns has now decreased in size, but not to any very considerable 

 extent externally, but is gradually decreasing internally. At the opposite 

 extremity to the anterior lobes of the embryo, the margin of the vitellus 

 may be observed as having broken into a series of very even cells 

 (PI. VII., fig. 6), transparent in colour and regular in position, forming 

 two or three very decided rows, until they gradually disappear in the 

 undeveloped structure of the vitellus. 



The external surface of these several rows of transparent cells appears 

 (PL VII., fig. 6) to be enclosed by a membrane of extreme tenuity, that 

 is evidently connected with and forms the outer walls of the alimentary 

 canal, al. The marginal cells appear to build up the fibrous structure of 

 the walls, while certain small particles of granular waste (gw) matter fall 

 into the central passage. Here they exist as foreign bodies of not any 

 large amount, and lie enclosed within a cavity of their own makinc ; 



