Sentence:  the other , a <WSD>shallow</WSD> concave gradient ( Fig. 1 ) , was produced with a so-called `` cone sphere '' apparatus , the `` cone '' being a 2 - liter Erlenmeyer flask and the `` sphere , '' a 2 - liter round-bottom flask . 
Meaning of the word marked with <WSD> tags:  lacking physical depth; having little spatial extension downward or inward from an outer surface or backward or outward from a center