EMBRYOLOGY OF CLEPSINE. 289 



with each other and with the marginal sinus^ as seen on the right 

 side of the figure. Between some of these channels other trans- 

 verse but smaller channels are often seen. The circulating fluid 

 is almost colourless, and contains numerous corpuscles. 



Bidder (|-|) states that both on the dorsal and the ventral 

 surface of each somatomere are found branches coming from the 

 marginal sinus. I have never observed such branches. Although 

 I have never been able to find any connection between the two 

 circulatory systems, I cannot of course say positively that there 

 is none. That there is no such communication as Leydig sup- 

 posed is perfectly certain. In regard to the nature of the lacunar 

 cavities, I fully adopt the ophiion maintained by Leuckart (-ji-°-^), 

 that they are parts of the laody-cavity. I am unable to state 

 anything definite in regard to the way in which the blood-vessels 

 originate. Fig. 96 represents a horizontal section, in wdiich are 

 seen parts of the suboesophageal ganglionic mass (s s g.) and two 

 body-ganglia. The intermediate ganglia lay beneath the plane 

 of this section. Just behind the suboesophageal ganglia is seen 

 a longitudinal collection of mesoderm cells, from which four pairs 

 of lateral branches take their departure. Judging from the 

 position of these cells and the number of the branches, it seems 

 quite possible that they represent the anterior end of the ventral 

 trunk, with its four pairs of tributary branches. Another inter- 

 pretation is, however, possible, namely, that these branches are 

 sections of the body-septa. 



Summary (Sections lY — VII). — I. The first meridional cleav- 

 age, passing from the oral to the aboral pole, divides the egg 

 into two unequal segments, the larger of which contains the 

 remains of the ring-discs. This segment includes the greater 

 part of the future ectoderm, the entire mesoderm, and about 

 one third of the entoderm ; the smaller segment contains two 

 thirds of the entoderm and a little ectoderm. 



2. The second meridional cleavage, passing from the centre 

 outwards, cuts off about one third of the larger segment, and 

 divides' the smaller into nearly equal parts. Thus four large 

 blastomeres are produced, three of which {a, c, h) are nearly of 

 the same size, and contain the entire entoderm ; the fourth and 

 larger blastomere {x) contains the entire mesoderm and a large 

 sliare of the ectoderm. 



3. The oral pole of each blastomere is split off as an ecto- 

 blast. The first ectoblast is produced by ;?•, the second by h, and 

 the third and fourth simultaneously by a and c. The four ecto- 

 blasts lie in cruciform order iu the boundary-lines of a, c, b, and 

 X. During the divisions of the primary neuroblast, other 

 ectoblasts are added to the four original ones from a, 1), and c. 



4. Thfe dorsal blastomere (,r) (so called because it is opposed 



