. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. 236 COMPARATIVE ANATOMY nit. —nil' (placodes) take part, in addition to the main ganglia already referred to. Two rows of these accessory ganglia (which probably corre- spond to the vestiges of primitive in- tegumentary sense-organs) may often be distinguished in the embryo in either side of the head—a dorso-lateral, and a more ventral row just above the gill-clefts: the former arc spoken of as the lateral ganglia, and the latter as the ventral or epibranchial ganglia, which are associated with transitory epibranchial sen
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. 236 COMPARATIVE ANATOMY nit. —nil' (placodes) take part, in addition to the main ganglia already referred to. Two rows of these accessory ganglia (which probably corre- spond to the vestiges of primitive in- tegumentary sense-organs) may often be distinguished in the embryo in either side of the head—a dorso-lateral, and a more ventral row just above the gill-clefts: the former arc spoken of as the lateral ganglia, and the latter as the ventral or epibranchial ganglia, which are associated with transitory epibranchial sense-organs. In their primitive superficial position the two rows are connected with one another and with the central organ by cellular cords. It is probable that the motor fibres of these mixed nerves (as well as the cerebral portion of XI) grow out secondarily from the brain into the primary ganglionic nerve-rudi- ments arising from the upper row of nerve-centres formed loy the bifurca- tion of the band-like spinal motor tract. From the ventral row, which lies in the same plane as the ventral horns of gray matter, arise the Illrd, IVth, Vlth, and Xllth nerves, as already It must be remembered that the head is primitively composed of a series of metameres (p. 75), and it is therefore important to ascertain, as far as is possible in the present state of our knowledge, to which individual metameres the different cerebral nerves belong. As already mentioned, the olfactory and optic nerves present cer- tain peculiarities which bring them under another category, and they will be treated of later in connection with the corresponding sensory organs. The following general summary gives a scheme of the prob- able primitive relations of the head-segments and cerebral nerves, 1 An additional pair of vestigial cerebral nerves has been shown to exist in Protopterus, Ceratodus, Anura, and many Elasmobranchs, arising in the region of the lamina terminalis, crossing the olfactor
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