[Muscle spindle distribution of masticatory and prevertebral muscles with special reference to histological properties of pouch and biological significance in red kangaroo, Macropus rufus].

The masticatory and prevertebral muscles of two embryos and four young including a newborn and the pouch of two mothers of the red kangaroo were examined microscopically. Furthermore, four older young of the red kangaroo were examined macroscopically to define the tooth eruption. The results obtained are as follows: 1. The initial muscle spindles are already observed in the prevertebral muscles of the newborn young of 21 mm in cranio-rump length (CRL) , the number of spindles being 223 (right side) 110 (left side) in the longus capitis, 81 (r) -70 (1) in the longus colli, 31 (r) -39 (1) in the rectus capitis lateralis and 5 (r) -9 (1) in the rectus capitis anterioris. 2. The initial muscle spindles are observed in the masticatory muscles of the young of 68 mm in CRL and 28 g in body weight, which is much delayed than in the prevertebral muscles; the number of spindles being 33 (right) -46 (left) in the masseter and 173 (r) -187 (1) in the temporal muscle. 3. The medial and lateral pterygoid muscles are devoid of the muscle spindles. The jaw opening muscles are also devoid of them. 4. In the frontal incisors there is seen the beginning of calcification of the dentine and enamel and in the molars there is seen the beginning of the dentinal calcification at the cuspal pulp in the older young of 125 mm in CRL and 86 g in body weight. 5. In the skin with less hair lining the inner surface of the pouch, abundant large sweat glands are well observed, especially surrounding the basal region of the nipple and in the pleat formation of the skin. 6. The lactiferous mammary gland is enlarged, the lobules being divided by the interlobular muscle fiber tissue and enwrapped by the muscular capsule. The milk is squirted by the glandular muscle contraction from the gland to the nipple, to which the baby attaches itself. 7. The musculature of the pouch wall is developed to form the sphincter muscle of the pouch orifice. The sphincter muscle would play an important role in conditioning the optimum temperature and moisture of the pouch for the naked baby inside the pouch.