Coincidentally with the appearance of a differentiation into a somatic and splanchnic layer the mesoblast plates become partially split by a series of transverse lines of division into protovertebræ. Only the proximal regions of the plates become split in this way, while their peripheral parts remain quite intact. As a result of this each plate becomes divided into a proximal portion adjoining the medullary canal, which is divided into _protovertebræ_, and may be called the _vertebral plate_, and a peripheral portion not so divided, which may be called the _lateral plate_. These two parts are at this stage quite continuous with each other; and, as will be seen in the sequel, the body-cavity originally extends uninterruptedly to the summit of the vertebral plates. By stage D at the least ten protovertebræ have appeared. In Torpedo the mesoblast commences to be divided into two layers much earlier than in Pristiurus; and even before stage C this division is more or less clearly marked. In the head and tail the condition of the mesoblast is by no means the same as in the body. In the tail the plates of mesoblast become considerably thickened and give rise to two projections, one on each side, which have already been alluded to as caudal or tail-swellings; vide Pl. 8, figs. D, F, and Pl. 10, fig. 3_f_ and fig. 4, _ts_. These masses of mesoblast are neither divided into protovertebræ, nor do they exhibit any trace of a commencing differentiation into somatopleure and splanchnopleure. In the head, so far as I have yet been able to observe, the mesoblastic plates do _not_ at this stage become divided into protovertebræ. The other changes exhibited in the cephalic region are of interest, mainly from the fact that here appears a cavity in the mesoblast directly continuous with the body-cavity (when that cavity becomes formed), but which appears at a very much earlier date than the body-cavity. This cavity can only be looked on in the light of a direct continuation of the body or peritoneal cavity into the head. Theoretical considerations with reference to it I propose reserving till I have described the changes which it undergoes in the subsequent periods. Pl. 10, figs. 3_a_, 3_b_ and 3_c_ exhibit very well the condition of the mesoblast in the head at this period. In fig. 3_c_, a section taken through the back part of the head, the mesoblast plates have nearly the same form as in the sections immediately behind. The ventral continuation of the mesoblast formed by the lateral plate has, however, become much thinner, and the dorsal or vertebral portion has acquired a more triangular form than in the sections through the trunk (figs. 3_d_ and 3_e_). In the section (fig. 3_b_) in front of this the ventral portion of the plate is no longer present, and only that part exists which corresponds with the vertebral division of the primitive plate of mesoblast. In this a distinct cavity, forming part of the body-cavity, has appeared.