EMBRYONIC-DEVELOPMENT OF THE HOUSE SHREW (SUNCUS-MURINUS) .2. EMBRYOS AT STAGES 11 AND 12 WITH 13 TO 29 PAIRS OF SOMITES, SHOWING LIMB BUD FORMATION AND CLOSED CEPHALIC NEURAL-TUBE

The embryonic development of the laboratory house shrew (Suncus murinus) was observed during the period from 13 to 29 pairs of somites and was compared with that of other mammalian species. This period ranged from 10 to 12 days after fertilization. Embryos were allocated to stages 11 A-C and 12 A-C according to somitic count. These embryos were characterized by the following features. 1. The curvature of the body became gradually enhanced, changing from a parenthesis-like to a bracket-like shape. 2. The swelling of the future hindlimb bud appeared at stage 11A, while the forelimb bud became apparent at stage 12A. The latter developed rapidly and had exceeded the former in size by the end of stage 12C. 3. The timing of closure of the cephalic neuropores varied, but the order of closure is constant. The prosencephalic neuropore closed later. The caudal neuropore still existed even at the end of stage 12C. 4. In all of the cranial nerve ganglia except for the auditory one, crest-derived cells seemed to contribute to the initial formation, while placodal cells seemed to be involved later. A different pattern of primordial formation was observed between trigeminal/facial and glossopharyngeal/vagal ganglia. 5. By the end of stage 12C, the eye cup and nasal pit began to form, and the endolymphatic appendage also appeared. 6. In the heart, the sinus valves were well developed, and the atrium and ventricle were being divided by the developing interatrial septum I and interventricular septum, respectively. 7. The thyroid primordium was temporarily everted and projected into the pharynx. The internal structure of this protruding thyroid resembled ''tuberculum thyroideum'' observed in the human embryo.