EARLY EMBRYOS. 99 



grown out conspicuously. The broad flattened trunk terminal of the preceding 

 embryo is here represented, and at a the anal region, a point anterior to which the 

 number of somites corresponds in a general way to that in the earlier stage. 

 Noteworthy advances include: 



(1) A more definite modeling of the regions of head and trunk. The latter 

 has now lifted up above the surrounding blastoderm, and the head (including the 

 chin region) has separated from the yolk-wall. 



(2) The gill-slits are now conspicuous, although, as sections show, they have 

 not yet broken through; we note that the spiracular slit s, evidently the equivalent 

 of g 1 in the former stage, is of considerable size; behind it occur three prominent 

 depressions and the trace of fourth and fifth. 



(3) The appearance of pronephros and pronephric duct; the pronephros itself is 

 situated at the plane of the ninth, tenth, eleventh and twelfth somites, as can 

 better be seen in the transparent preparation in the same embryo, plate vn, fig. 43. 



(4) The knob-like terminal eminence of the head region has greatly increased 

 in size. 



A fourth embryo of this period is shown as a transparent preparation in plate 

 vir, fig. 44. It contains a greater number of somites than the preceding, over 80 

 as opposed to over 60, but in many regards it appears to be less advanced in devel- 

 opment. Thus we note that its head region appears somewhat less mature than in 

 the former embryo; the chin is less definitely established and so also the gill-slits 

 are shallower and the optic and auditory vesicles and the pronephros less definite. 

 The tail, moreover, is less pointed, even bulbous where the terminal growth is taking 

 place. In this stage we note the presence of a conspicuous postanal gut. The 

 details of the vascular supply of the gill region are well shown ; the spiracular artery 

 is conspicuous, and, further hindward, we observe the duct of the pronephros (pnd) 

 and the postanal gut pag. 



A series of characteristic sections of this stage is given in figures 76 A-N. 

 These show a general correspondence to the conditions of the young shark. In 

 fig. c the premandibular head cavity (pm) is shown; in D the mandibular (m). In 

 this section also we observe that the mouth has not yet broken through. In later 

 sections, as in E, F, H, i, and K, we note that the gill-slits have not been com- 

 pleted; fusions of the gut wall with the ectoderm have, however, occurred. We 

 note in section M, passing through the pronephric tubules, that the relation of these 

 structures corresponds closely to that in the young shark. A subnotochordal rod, 

 conspicuous in the earlier stage, is here represented only in a rudimentary condition, 

 as in N ; the gut has separated from the notochord and the main vascular trunks now 

 appear in the region formerly occupied by the subnotochordal rod. The present 

 stage corresponds closely with that of the shark in which the mesoblast bounds a 

 continuous myo-, nephro-, and splanchno-ccele. In Chimera, however, continuity 

 in these regions is less clearly marked, a feature which evinces greater develop- 

 mental specialization, i. e. , in masking an archaic condition and preparing the way 

 for the prompter growth of structures useful to the young fish. 



