DEPARTMENT OF EMBRYOLOGY. 87 



I may add that as an index of head size a modulus was selected, con- 

 sisting of the mean of the greatest horizontal circumference and the 

 biauricular transverse arc. The head modulus at first is nearly equiva- 

 lent to the sitting height (96 per cent) . Towards the end of pregnancy 

 it averages 16 per cent less than the sitting height. In a few outlying 

 instances the head modulus equals or exceeds the sitting height. These 

 are cases showing distention of the soft tissues of the scalp. If, under 

 the usual conditions, the head modulus is greater than the sitting height, 

 it should be regarded as abnormal. In this way we may be able to 

 detect early cases of hydrocephalus. If, on the other hand, the head 

 modulus is too small, we have an indication of microcephalus. 



CYTOLOGY. 



Our report for the year 1918 described the observations of M. R. 

 Lewis and W. H. Lewis on the contraction phenomenon of smooth- 

 muscle cells in hanging-drop preparations of the amnion of the chick. 

 This method of studying the mechanism of muscle contraction has 

 proved of great value, as it obviates the necessity of considering the 

 complicated cell architectm^e of adult muscle. It brings the phe- 

 nomenon within the confines of a single and quite simple cell which can 

 be directly observed in the living condition throughout the experimen- 

 tation. Further observations have since been made by M. R. Lewis 

 on smooth-muscle cells of the amnion, and the study has been extended 

 to include (1) the beating heart of 2 to 3-day chick embryos; (2) prepa- 

 rations of teased heart-muscle fibers and skeletal-muscle fibers of chick 

 embryos; (3) marine copepods, the cross-striated muscle fibers of which 

 could be studied while the animal remained alive; (4) isolated sarco- 

 styles from the wing muscle of the house fly; and (5) control prepara- 

 tions of muscle fibers from adult dogs, cats, and turtles. 



]Mrs. Lewis finds that among the muscle cells of tissue-culture growth, 

 whether originating from the amnion, the heart, or the skeletal muscles, 

 there can ahvays be found isolated embryonic muscle cells capable of 

 contraction. The cytoplasm of these cells is marked by a higher re- 

 fraction, but otherwise does not appear to be different from the cyto- 

 plasm of other kinds of cells in the culture. The contraction exhibited 

 by each muscle cell varies in some details, according to the tj^DC of tis- 

 sue from which it arises. The fundamental process, however, is the 

 same for the different tj^pes of muscle cells; i. e., at a certain point 

 within the protoplasm of the cell some change takes place which results 

 in a flow of protoplasm towards this region and a consequent thicken- 

 ing and shortening of the area involved. A neutralization of the 

 active change then occurs, accompanied by a relaxation and retiorn 

 of the protoplasm to its normal position. 



Nothing more definite as to the nature of the contraction phe- 

 nomenon can be said for the present; but from these observations 



