CONTRIBUTIONS TO THE EMBRYOLOGY OF NEMERTEA. 443 
in the cavity where we saw them originate—the archiccel (l.c. 
(30), pl. v, figs. 75—79). This explanation is again far more 
natural than another, which would attribute the relative 
situation of brain-lobes and blood lacuna to a later develop- 
ment of the blood system by which it came to surround these 
lobes. 
A third possibility, viz. that the blood system of these 
animals has arisen by a process of hollowing out of solid cell- 
rows and cell-groups, must be emphatically rejected on the 
authority of numerous preparations. 
In the midst of the outer layer of longitudinal muscle-fibres 
the connective tissue affects another important character. It 
here remains visible as a continuous cylindrical sheath sepa- 
rating an outer from an inner layer of these fibres (1.c. (30), 
pl. v, figs. 88 and 89), and distantly reminds one of the 
layer of cambium tissue in dicotyledonous plants. I mention 
this name because I feel assured that the increase in thick- 
ness of this muscular layer is for the greater part due toa 
gradual passage of cells from this more neutral layer into 
longitudinal muscle-fibres. The meade of origin of the first 
muscle-fibres in the embryo would be thus continued in the 
older larvz, for it must be remarked that the arrangement just 
mentioned can only be observed in young animals that have 
already attained a few millimetres in length, and has again 
disappeared in the adults. 
With respect to the circular and internal longitudinal mus- 
cular layers, which have been already described as appearing 
later than the outer longitudinal muscles, it must still be noted 
that they only reach as far forwards as the posterior brain- 
lobes. The circular layer disappears in the fold between the 
posterior and anterior upper brain-lobes. It is noteworthy 
that this spot corresponds to the region of coalescence between 
the cephalic and the ventral plates, which at the same time 
may be said to mark off a prostomial from a metastomial 
region. Barrois has already called attention to this pheno- 
menon. 
We have now finished discussing the different derivatives of 
