ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 153 



The sarcolemma is not prolonged through the tendon or over the tendon 

 in either type of muscle. The sarcoplasm does not at any place in 

 either type of muscle pass through the sarcolemma. 



Transverse Segmentation and Differentiation of Chromosomes.* 

 W. E. Agar has studied this in the spermatogenesis of Lejpido siren, and 

 gives the following summary of his results. 1. The tendency for 

 chromosomes to become transversely segmented or constricted is a 

 widespread characteristic. It becomes operative especially, but not 

 solely, whenever the chromosomes are short in comparison with their 

 length, as happens normally in meiosis, and exceptionally in somatic 

 tissues. 2. The point at which the constriction or segmentation takes 

 place in any given chromosome is constant for that chromosome, and is 

 the same as the point at which it most readily bends to form the angle 

 of the V when present in that form. 3. The constancy of the position 

 at which transverse segmentation takes place, indicates a constant 

 differentiation of the chromosomes in a lengthwise direction. 4. The 

 presence of transverse constrictions or joints in chromosomes, ^s 

 not, without special evidence, to be taken as indication of bivalence, 

 or of a future division plane. 



Origin of Mitochrondria in Sex-cells. — Wilke brings forward a 

 number of cases which go to show that the view that the mitochondria 

 have a nuclear origin must not be regarded as securely demonstrated. 

 There are many cases where origin from the nucleus seems highly im- 

 probable. In the spermatogenesis of Hydrometra laciistris, the evidence 

 seems clearly in favour of the plasmic origin of the mitochondria. 



Formation of Hssmoglobin.! — Herm. Schridde has studied this 

 question in the bone marrow of young rabbits, which is rich in baso- 

 philous erythroblasts. To granules and threads (plastosomes) in these 

 erythroblasts the formation of the haemoglobin is demonstrably 

 referable. 



Osteoblasts and Osteoclasts. § — A. C. Geddes has investigated the 

 earlier stages of the cytomorphosis of the osteoblasts and the origin of 

 the osteoclasts. His material was chiefly from the human embryo, but 

 frequent reference was made to pig, chick, axolotl, and trout. 



He finds evidence which makes it probable, though by no means 

 certain, that bone is derived from the ectoderm, not from the mesoderm. 

 Osteoblasts arise from the cells of the ectoderm, and migrate as 

 individuals to the sites of bone formation, passing through the perios- 

 teum en route. Adult osteoblasts do not form a syncytium. The cell- 

 processes which are lodged in the canaliculi, though frequently passing 

 through lacunae occupied by other cells, extend uninterruptedly to the 

 Haversian canal of their system. 



Periosteum, far from being an osteogenetic membrane, is a limiter 



* Quart. Journ. Micr. Sci., Iviii. (1912) pp. 285-98 (2 pis.). 



t Anat. Anzeig., xlii. (1912) pp. 199-506 (4 figs.). 



t Anat. Anzeig., xlii. (1912) pp. 511-17 (1 fig.). 



§ Journ. Anat. Physiol., xlvii. (1912) pp. 159-76 (12 figs.). 



Ajrril 16th, WIS M 



