GLOSSARY U77 



SECTION CBCSS - cut made at right angles to the long axla of the embryo. Syn. , tranaverse section. 

 SECTION, FFONTAL - cut made parallel to the longitudinal axis of the embryo and separating the more dorsal 



from the more ventral. Syn., horizontal section. 

 SECTION, SAGITTAL - out made parallel to the longitudinal axis of the embryo but separating the right from 



the left portions. Term often confused with "median" or "longitudinal" which really mean no more 



than "axial," hence could also be "frontal". 

 SECTIONS, SEEIAL - thin (microscopic) slices of an embryo laid on the slide in sequence (generally from 



left to right, as one reads) so that the beginning of the embryo is at one side (left) and the end 



of the embryo at the opposite side (right) of the slide. 

 SEGMENTATION - term used synonymously with cleavage. Also means serial repetition of embryonic rudinenta 



(structural patterns) in successive levels of regular spacing, as In the case of somites, and spinal 



nerves. Syn., cleavage. 

 SEGREGATION - the separation of self-differentiating embryonic rudiments; the organizational process of 



embryogeny; autonomlzlng (Weiss); the aggregation of various spatial systems Independent of each other 



and leading to self-differentiating potentialities. Originally used (Ray Lankaater) in discussing the 



gastrea theory to mean a separation of the physiological molecules that are going to form ecto- and 



endoderm. 

 SEGREGATION, EMBRYONIC - progressive restriction of original potencies In the embryo; the process of step 



by step repartltlonlng of the originally homogeneous zygote Into the separate parts of the presump- 

 tive embryo. 

 SEGREGATION, PRECOCIOUS - segregation found In mosaic eggs where local differences arise even before 



cleavage and a minimum of modification in response to any Internal environmental factors occurs In 



subsequent development (Lankester, 1877). 

 SELBSTOEGANIZATION - invisible process of construction and reconstruction of a normal blastema, with Its 



quantitative organization gradient which is itself the basis for the segregation from qualitatively 



differing organ- forming regions. 

 SELF-DIFFERENTIATING CAPACITY - the capacity of a part of a developing system to pursue a specific course. 



The characters of that course are determined by intrinsic properties of the part (Roux, I88I). 



There can be no self-differentiation without prior induction. (See differentiation, self.) 

 SELF- ORGANIZATION - obsolete term which meant the alleged appearance of a lens without the stimulus normal- 

 ly coming from the optic cup (see double assurance). 

 SELFWISE - behavior of a transplant in a manner expected in its original environment. In accordance with 



its normal prospective significance. 

 SENESCENCE - the progressive loss of growth power; old age. 



SENSITIZATION THEORY - calcium Is the true activating agent in artificial parthenogenesis and other sub- 

 stances increase the permeability of the egg cortex to calcium (Pasteels). 

 SENSORY LOAD - determined by the number of receptor organs associated with a specific nerve. 

 SEX, HETERODYNAMIC - the sex in which the gametes are of two klnda with respect to the possession of 



specific sex influencing chromosomes, such as the X-chromosome in Drosophila. The frog and human 



male are presumably heterogametic. 

 SIGNIFICANCE, PROSPECTIVE - actual fate of any part of the original egg. Syn., Drlesch's "prospektive 



Bedeutung" . 

 SITUS INVERSUS - an inversion of the bilateral symmetry; reversal of right and left symmetry. 

 SITUS INVERSUS VISCERUM - twisting of the digestive tract and sometimes the heart, occurring naturally 



(rarely) or as a result of shifting of embryonic parts (Spemann, I906) as In reversing a square 



piece of presumptive neural plate and archenteron of the early gastrula. 

 SOL - a colloidal system In which the particles of a solid or of a second liquid are suspended In a con- 

 tinuous phase of a liquid, the particles or their aggregates being too large to go through animal 



membranes rapidly or at all. 

 SOMATIC DOUBLING - doubling of the initial number of chromosomes with which the egg begins development, 



occurring (probably In most cases) at the first or early mitotic divisions (cleavages) of the egg, 



after fertilization. 

 SOMATOBLAST - blastomere with specific germ layer predisposition such as actodermal somatoblasts. 

 SPALTUNG - (German) fusion of posterior neural axes in a twin embryo, simulating an induction. 

 SPECIFICITY - the summation of the cytochemlcal characteristics of different protoplasms (Humphrey and 



Burns, 1939)- 

 SPERMOPHILE GROUP - portion of the amboceptor in Lillie's fertlllzln hypothesis into which sperm recep- 

 tors fit in the fertilization reaction. 

 SPERM RECEPTOR - chemical group associated with the spermatozoa, reacting with fertlllzln (amboceptor) 



in Lillie's side chain hypothesis of the fertllizin reaction. 

 SPINA BIFIDA - split tall caused by a variety of abnormal environmental conditions such as heat, cold, 



lack of oxygen, centrlfugation, any of which may prevent the proper closure of the blastopore which 



leads to this split-tailed condition. 

 STEPWISE INHIBITION - successive inhibitions of organic processes by successively stronger applications 



of external agents. 

 STEREOBLASTULA - solid blastula experimentally produced by subjecting (Echlnoderm) eggs to alkaloids; 



normal blastocoel filled with solid mass of cells (e.g., Crepldula). 

 STERILITY, SELF - Inability of eggs and sperm of the same (hermaphroditic) Individual to fuse and give 



rise to an embryo (e.g., Ciona Intestlnalls, an Ascidian) . 

 STERNOPAGUS - sternal union of conjoined twins. 

 STICOTHOPISM - faculty of acquiring and losing clavlform shape of the bottle cells of the blastoporal 



lip during gastrulatlon (Ruffini, 1925). 



